diff --git a/.gitignore b/.gitignore
index 423e89b..588b9f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1 @@
-*.gng
+.goat
diff --git a/.go.yaml b/.go.yaml
new file mode 100644
index 0000000..e82fa5a
--- /dev/null
+++ b/.go.yaml
@@ -0,0 +1,2 @@
+---
+path: github.com/mediocregopher/ginger
diff --git a/seq/hashmap.go b/seq/hashmap.go
new file mode 100644
index 0000000..92af30b
--- /dev/null
+++ b/seq/hashmap.go
@@ -0,0 +1,125 @@
+package seq
+
+import (
+	"fmt"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Hash maps are built on top of hash sets. KeyVal implements Setable, but the
+// Hash and Equal methods only apply to the key and ignore the value.
+
+// Container for a key/value pair, used by HashMap to hold its data
+type KV struct {
+	Key types.Elem
+	Val types.Elem
+}
+
+func KeyVal(key, val types.Elem) *KV {
+	return &KV{key, val}
+}
+
+// Implementation of Hash for Setable. Only actually hashes the Key field
+func (kv *KV) Hash(i uint32) uint32 {
+	return hash(kv.Key, i)
+}
+
+// Implementation of Equal for Setable. Only actually compares the key field. If
+// compared to another KV, only compares the other key as well.
+func (kv *KV) Equal(v types.Elem) bool {
+	if kv2, ok := v.(*KV); ok {
+		return equal(kv.Key, kv2.Key)
+	}
+	return equal(kv.Key, v)
+}
+
+// Implementation of String for Stringer
+func (kv *KV) String() string {
+	return fmt.Sprintf("%v -> %v", kv.Key, kv.Val)
+}
+
+// HashMaps are actually built on top of Sets, just with some added convenience
+// methods for interacting with them as actual key/val stores
+type HashMap struct {
+	set *Set
+}
+
+// Returns a new HashMap of the given KVs (or possibly just an empty HashMap)
+func NewHashMap(kvs ...*KV) *HashMap {
+	ints := make([]types.Elem, len(kvs))
+	for i := range kvs {
+		ints[i] = kvs[i]
+	}
+	return &HashMap{
+		set: NewSet(ints...),
+	}
+}
+
+// Implementation of FirstRest for Seq interface. First return value will
+// always be a *KV or nil. Completes in O(log(N)) time.
+func (hm *HashMap) FirstRest() (types.Elem, Seq, bool) {
+	if hm == nil {
+		return nil, nil, false
+	}
+	el, nset, ok := hm.set.FirstRest()
+	return el, &HashMap{nset.(*Set)}, ok
+}
+
+// Returns a new HashMap with the given value set on the given key. Also returns
+// whether or not this was the first time setting that key (false if it was
+// already there and was overwritten). Has the same complexity as Set's SetVal
+// method.
+func (hm *HashMap) Set(key, val types.Elem) (*HashMap, bool) {
+	if hm == nil {
+		hm = NewHashMap()
+	}
+
+	nset, ok := hm.set.SetVal(KeyVal(key, val))
+	return &HashMap{nset}, ok
+}
+
+// Returns a new HashMap with the given key removed from it. Also returns
+// whether or not the key was already there (true if so, false if not). Has the
+// same time complexity as Set's DelVal method.
+func (hm *HashMap) Del(key types.Elem) (*HashMap, bool) {
+	if hm == nil {
+		hm = NewHashMap()
+	}
+
+	nset, ok := hm.set.DelVal(KeyVal(key, nil))
+	return &HashMap{nset}, ok
+}
+
+// Returns a value for a given key from the HashMap, along with a boolean
+// indicating whether or not the value was found. Has the same time complexity
+// as Set's GetVal method.
+func (hm *HashMap) Get(key types.Elem) (types.Elem, bool) {
+	if hm == nil {
+		return nil, false
+	} else if kv, ok := hm.set.GetVal(KeyVal(key, nil)); ok {
+		return kv.(*KV).Val, true
+	} else {
+		return nil, false
+	}
+}
+
+// Same as FirstRest, but returns values already casted, which may be convenient
+// in some cases.
+func (hm *HashMap) FirstRestKV() (*KV, *HashMap, bool) {
+	if el, nhm, ok := hm.FirstRest(); ok {
+		return el.(*KV), nhm.(*HashMap), true
+	} else {
+		return nil, nil, false
+	}
+}
+
+// Implementation of String for Stringer interface
+func (hm *HashMap) String() string {
+	return ToString(hm, "{", "}")
+}
+
+// Returns the number of KVs in the HashMap. Has the same complexity as Set's
+// Size method.
+func (hm *HashMap) Size() uint64 {
+	return hm.set.Size()
+}
diff --git a/seq/hashmap_test.go b/seq/hashmap_test.go
new file mode 100644
index 0000000..6b0368f
--- /dev/null
+++ b/seq/hashmap_test.go
@@ -0,0 +1,115 @@
+package seq
+
+import (
+	. "testing"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+func kvints(kvs ...*KV) ([]*KV, []types.Elem) {
+	ints := make([]types.Elem, len(kvs))
+	for i := range kvs {
+		ints[i] = kvs[i]
+	}
+	return kvs, ints
+}
+
+// Test creating a Set and calling the Seq interface methods on it
+func TestHashMapSeq(t *T) {
+	kvs, ints := kvints(
+		KeyVal(1, "one"),
+		KeyVal(2, "two"),
+	)
+
+	// Testing creation and Seq interface methods
+	m := NewHashMap(kvs...)
+	ms := testSeqNoOrderGen(t, m, ints)
+
+	// ms should be empty at this point
+	assertEmpty(ms, t)
+}
+
+// Test getting values from a HashMap
+func TestHashMapGet(t *T) {
+	kvs := []*KV{
+		KeyVal(1, "one"),
+		KeyVal(2, "two"),
+	}
+
+	// Degenerate case
+	m := NewHashMap()
+	assertEmpty(m, t)
+	v, ok := m.Get(1)
+	assertValue(v, nil, t)
+	assertValue(ok, false, t)
+
+	m = NewHashMap(kvs...)
+	v, ok = m.Get(1)
+	assertSeqContentsHashMap(m, kvs, t)
+	assertValue(v, "one", t)
+	assertValue(ok, true, t)
+
+	v, ok = m.Get(3)
+	assertSeqContentsHashMap(m, kvs, t)
+	assertValue(v, nil, t)
+	assertValue(ok, false, t)
+}
+
+// Test setting values on a HashMap
+func TestHashMapSet(t *T) {
+
+	// Set on empty
+	m := NewHashMap()
+	m1, ok := m.Set(1, "one")
+	assertEmpty(m, t)
+	assertSeqContentsHashMap(m1, []*KV{KeyVal(1, "one")}, t)
+	assertValue(ok, true, t)
+
+	// Set on same key
+	m2, ok := m1.Set(1, "wat")
+	assertSeqContentsHashMap(m1, []*KV{KeyVal(1, "one")}, t)
+	assertSeqContentsHashMap(m2, []*KV{KeyVal(1, "wat")}, t)
+	assertValue(ok, false, t)
+
+	// Set on second new key
+	m3, ok := m2.Set(2, "two")
+	assertSeqContentsHashMap(m2, []*KV{KeyVal(1, "wat")}, t)
+	assertSeqContentsHashMap(m3, []*KV{KeyVal(1, "wat"), KeyVal(2, "two")}, t)
+	assertValue(ok, true, t)
+
+}
+
+// Test deleting keys from sets
+func TestHashMapDel(t *T) {
+
+	kvs := []*KV{
+		KeyVal(1, "one"),
+		KeyVal(2, "two"),
+		KeyVal(3, "three"),
+	}
+	kvs1 := []*KV{
+		KeyVal(2, "two"),
+		KeyVal(3, "three"),
+	}
+
+	// Degenerate case
+	m := NewHashMap()
+	m1, ok := m.Del(1)
+	assertEmpty(m, t)
+	assertEmpty(m1, t)
+	assertValue(ok, false, t)
+
+	// Delete actual key
+	m = NewHashMap(kvs...)
+	m1, ok = m.Del(1)
+	assertSeqContentsHashMap(m, kvs, t)
+	assertSeqContentsHashMap(m1, kvs1, t)
+	assertValue(ok, true, t)
+
+	// Delete it again!
+	m2, ok := m1.Del(1)
+	assertSeqContentsHashMap(m1, kvs1, t)
+	assertSeqContentsHashMap(m2, kvs1, t)
+	assertValue(ok, false, t)
+
+}
diff --git a/seq/hashset.go b/seq/hashset.go
new file mode 100644
index 0000000..5923d3e
--- /dev/null
+++ b/seq/hashset.go
@@ -0,0 +1,414 @@
+package seq
+
+import (
+	"fmt"
+	"hash/crc32"
+	"reflect"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// This is an implementation of a persistent tree, which will then be used as
+// the basis for vectors, hash maps, and hash sets.
+
+type Setable interface {
+
+	// Returns an integer for the value. For two equivalent values (as defined
+	// by ==) Hash(i) should always return the same number. For multiple values
+	// of i, Hash should return different values if possible.
+	Hash(uint32) uint32
+
+	// Given an arbitrary value found in a Set, returns whether or not the two
+	// are equal
+	Equal(types.Elem) bool
+}
+
+// Returns an arbitrary integer for the given value/iteration tuple
+func hash(v types.Elem, i uint32) uint32 {
+	switch vt := v.(type) {
+
+	case Setable:
+		return vt.Hash(i) % ARITY
+
+	case uint:
+		return uint32(vt) % ARITY
+	case uint8:
+		return uint32(vt) % ARITY
+	case uint32:
+		return uint32(vt) % ARITY
+	case uint64:
+		return uint32(vt) % ARITY
+	case int:
+		return uint32(vt) % ARITY
+	case int8:
+		return uint32(vt) % ARITY
+	case int16:
+		return uint32(vt) % ARITY
+	case int32:
+		return uint32(vt) % ARITY
+	case int64:
+		return uint32(vt) % ARITY
+	case float32:
+		return uint32(vt) % ARITY
+	case float64:
+		return uint32(vt) % ARITY
+
+	case string:
+		return crc32.ChecksumIEEE([]byte(vt)) % ARITY
+
+	case []byte:
+		return crc32.ChecksumIEEE(vt) % ARITY
+
+	default:
+		err := fmt.Sprintf("%s not hashable", reflect.TypeOf(v))
+		panic(err)
+	}
+}
+
+// Returns whether two values (potentially Setable's) are equivalent
+func equal(v1, v2 types.Elem) bool {
+	if v1t, ok := v1.(Setable); ok {
+		return v1t.Equal(v2)
+	} else if v2t, ok := v2.(Setable); ok {
+		return v2t.Equal(v1)
+	} else if v1t, ok := v1.([]byte); ok {
+		if v2t, ok := v2.([]byte); ok {
+			if len(v1t) != len(v2t) {
+				return false
+			}
+			for i := range v1t {
+				if v1t[i] != v2t[i] {
+					return false
+				}
+			}
+			return true
+		}
+		return false
+	} else {
+		return v1 == v2
+	}
+}
+
+// The number of children each node in Set (implemented as a hash tree) can have
+const ARITY = 32
+
+// A Set is an implementation of Seq in the form of a persistant hash-tree. All
+// public operations on it return a new, immutable form of the modified
+// variable, leaving the old one intact. Immutability is implemented through
+// node sharing, so operations aren't actually copying the entire hash-tree
+// everytime, only the nodes which change, making the implementation very
+// efficient compared to just copying.
+//
+// Items in sets need to be hashable and comparable. This means they either need
+// to be some real numeric type (int, float32, etc...), string, []byte, or
+// implement the Setable interface.
+type Set struct {
+
+	// The value being held
+	val types.Elem
+
+	// Whether or not the held value has been set yet. Needed because the value
+	// could be nil
+	full bool
+
+	// Slice of kids of this node. Could be an empty slice
+	kids []*Set
+
+	// Number of values in this Set.
+	size uint64
+}
+
+// Returns a new Set of the given elements (or no elements, for an empty set)
+func NewSet(vals ...types.Elem) *Set {
+	if len(vals) == 0 {
+		return nil
+	}
+	set := new(Set)
+	for i := range vals {
+		set.setValDirty(vals[i], 0)
+	}
+	set.size = uint64(len(vals))
+	return set
+}
+
+// Methods marked as "dirty" operate on the node in place, and potentially
+// change it or its children.
+
+// Dirty. Tries to set the val on this Set node, or initialize the kids slice if
+// it can't. Returns whether or not the value was set and whether or not it was
+// already set.
+func (set *Set) shallowTrySetOrInit(val types.Elem) (bool, bool) {
+	if !set.full {
+		set.val = val
+		set.full = true
+		return true, false
+	} else if equal(set.val, val) {
+		set.val = val
+		set.full = true
+		return true, true
+	} else if set.kids == nil {
+		set.kids = make([]*Set, ARITY)
+	}
+	return false, false
+}
+
+// dirty (obviously). Sets a value on this node in place. Only used during
+// initialization.
+func (set *Set) setValDirty(val types.Elem, i uint32) {
+	if ok, _ := set.shallowTrySetOrInit(val); ok {
+		return
+	}
+
+	h := hash(val, i)
+	if kid := set.kids[h]; kid != nil {
+		kid.setValDirty(val, i+1)
+	} else {
+		set.kids[h] = NewSet(val)
+	}
+}
+
+// Returns a copy of this set node, including allocating and copying the kids
+// slice.
+func (set *Set) clone() *Set {
+	var newkids []*Set
+	if set.kids != nil {
+		newkids = make([]*Set, ARITY)
+		copy(newkids, set.kids)
+	}
+	cs := &Set{
+		val:  set.val,
+		full: set.full,
+		kids: newkids,
+		size: set.size,
+	}
+	return cs
+}
+
+// The actual implementation of SetVal, because we need to pass i down the stack
+func (set *Set) internalSetVal(val types.Elem, i uint32) (*Set, bool) {
+	if set == nil {
+		return NewSet(val), true
+	}
+	cset := set.clone()
+	if ok, prev := cset.shallowTrySetOrInit(val); ok {
+		return cset, !prev
+	}
+
+	h := hash(val, i)
+	newkid, ok := cset.kids[h].internalSetVal(val, i+1)
+	cset.kids[h] = newkid
+	return cset, ok
+}
+
+// Returns a new Set with the given value added to it. Also returns whether or
+// not this is the first time setting this value (false if it was already there
+// and was overwritten). Completes in O(log(N)) time.
+func (set *Set) SetVal(val types.Elem) (*Set, bool) {
+	nset, ok := set.internalSetVal(val, 0)
+	if ok {
+		nset.size++
+	}
+	return nset, ok
+}
+
+// The actual implementation of DelVal, because we need to pass i down the stack
+func (set *Set) internalDelVal(val types.Elem, i uint32) (*Set, bool) {
+	if set == nil {
+		return nil, false
+	} else if set.full && equal(val, set.val) {
+		cset := set.clone()
+		cset.val = nil
+		cset.full = false
+		return cset, true
+	} else if set.kids == nil {
+		return set, false
+	}
+
+	h := hash(val, i)
+	if newkid, ok := set.kids[h].internalDelVal(val, i+1); ok {
+		cset := set.clone()
+		cset.kids[h] = newkid
+		return cset, true
+	}
+	return set, false
+}
+
+// Returns a new Set with the given value removed from it and whether or not the
+// value was actually removed. Completes in O(log(N)) time.
+func (set *Set) DelVal(val types.Elem) (*Set, bool) {
+	nset, ok := set.internalDelVal(val, 0)
+	if ok && nset != nil {
+		nset.size--
+	}
+	return nset, ok
+}
+
+// The actual implementation of GetVal, because we need to pass i down the stack
+func (set *Set) internalGetVal(val types.Elem, i uint32) (types.Elem, bool) {
+	if set == nil {
+		return nil, false
+	} else if set.full && equal(val, set.val) {
+		return set.val, true
+	} else if set.kids == nil {
+		return nil, false
+	}
+
+	h := hash(val, i)
+	return set.kids[h].internalGetVal(val, i+1)
+}
+
+// Returns a value from the Set, along with  a boolean indiciating whether or
+// not the value was found. Completes in O(log(N)) time.
+func (set *Set) GetVal(val types.Elem) (types.Elem, bool) {
+	return set.internalGetVal(val, 0)
+}
+
+// Actual implementation of FirstRest. Because we need it to return a *Set
+// instead of Seq for one case.
+func (set *Set) internalFirstRest() (types.Elem, *Set, bool) {
+	if set == nil {
+		return nil, nil, false
+	}
+
+	if set.kids != nil {
+		var el types.Elem
+		var rest *Set
+		var ok bool
+		for i := range set.kids {
+			if el, rest, ok = set.kids[i].internalFirstRest(); ok {
+				cset := set.clone()
+				cset.kids[i] = rest
+				return el, cset, true
+			}
+		}
+	}
+
+	// We're not nil, but we don't have a value and no kids had values. We might
+	// as well be nil.
+	if !set.full {
+		return nil, nil, false
+	}
+
+	return set.val, nil, true
+}
+
+// Implementation of FirstRest for Seq interface. Completes in O(log(N)) time.
+func (set *Set) FirstRest() (types.Elem, Seq, bool) {
+	el, restSet, ok := set.internalFirstRest()
+	if ok && restSet != nil {
+		restSet.size--
+	}
+	return el, Seq(restSet), ok
+}
+
+// Implementation of String for Stringer interface
+func (set *Set) String() string {
+	return ToString(set, "#{", "}#")
+}
+
+// Returns the number of elements in the Set. Completes in O(1) time.
+func (set *Set) Size() uint64 {
+	if set == nil {
+		return 0
+	}
+	return set.size
+}
+
+// Returns a Set with all of the elements of the original Set along with
+// everything in the given Seq. If an element is present in both the Set and the
+// Seq, the element in the Seq overwrites. Completes in O(M*log(N)), with M
+// being the number of elements in the Seq and N the number of elements in the
+// Set
+func (set *Set) Union(s Seq) *Set {
+	if set == nil {
+		return ToSet(s)
+	}
+
+	cset := set.clone()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); !ok {
+			return cset
+		} else if cset, ok = cset.SetVal(el); ok {
+			cset.size++
+		}
+	}
+}
+
+// Returns a Set with all of the elements in Seq that are also in Set. Completes
+// in O(M*log(N)), with M being the number of elements in the Seq and N the
+// number of elements in the Set
+func (set *Set) Intersection(s Seq) *Set {
+	if set == nil {
+		return nil
+	}
+
+	iset := NewSet()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); !ok {
+			return iset
+		} else if _, ok = set.GetVal(el); ok {
+			iset, _ = iset.SetVal(el)
+		}
+	}
+}
+
+// Returns a Set of all elements in the original Set that aren't in the Seq.
+// Completes in O(M*log(N)), with M being the number of elements in the Seq and
+// N the number of elements in the Set
+func (set *Set) Difference(s Seq) *Set {
+	if set == nil {
+		return nil
+	}
+
+	cset := set.clone()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); !ok {
+			return cset
+		} else {
+			cset, _ = cset.DelVal(el)
+		}
+	}
+}
+
+// Returns a Set of all elements that are either in the original Set or the
+// given Seq, but not in both. Completes in O(M*log(N)), with M being the number
+// of elements in the Seq and N the number of elements in the Set.
+func (set *Set) SymDifference(s Seq) *Set {
+	if set == nil {
+		return ToSet(s)
+	}
+
+	cset := set.clone()
+	var cset2 *Set
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); !ok {
+			return cset
+		} else if cset2, ok = cset.DelVal(el); ok {
+			cset = cset2
+		} else {
+			cset, _ = cset.SetVal(el)
+		}
+	}
+}
+
+// Returns the elements in the Seq as a set. In general this completes in
+// O(N*log(N)) time (I think...). If the given Seq is already a Set it will
+// complete in O(1) time. If it is a HashMap it will complete in O(1) time, and
+// the resultant Set will be comprised of all KVs
+func ToSet(s Seq) *Set {
+	if set, ok := s.(*Set); ok {
+		return set
+	} else if hm, ok := s.(*HashMap); ok {
+		return hm.set
+	}
+	vals := ToSlice(s)
+	return NewSet(vals...)
+}
diff --git a/seq/hashset_test.go b/seq/hashset_test.go
new file mode 100644
index 0000000..75872f8
--- /dev/null
+++ b/seq/hashset_test.go
@@ -0,0 +1,245 @@
+package seq
+
+import (
+	. "testing"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Test creating a Set and calling the Seq interface methods on it
+func TestSetSeq(t *T) {
+	ints := []types.Elem{1, "a", 5.0}
+
+	// Testing creation and Seq interface methods
+	s := NewSet(ints...)
+	ss := testSeqNoOrderGen(t, s, ints)
+
+	// ss should be empty at this point
+	s = ToSet(ss)
+	var nilpointer *Set
+	assertEmpty(s, t)
+	assertValue(s, nilpointer, t)
+	assertValue(len(ToSlice(s)), 0, t)
+}
+
+// Test setting a value on a Set
+func TestSetVal(t *T) {
+	ints := []types.Elem{0, 1, 2, 3, 4}
+	ints1 := []types.Elem{0, 1, 2, 3, 4, 5}
+
+	// Degenerate case
+	s := NewSet()
+	assertEmpty(s, t)
+	s, ok := s.SetVal(0)
+	assertSeqContentsSet(s, []types.Elem{0}, t)
+	assertValue(ok, true, t)
+
+	s = NewSet(ints...)
+	s1, ok := s.SetVal(5)
+	assertSeqContentsSet(s, ints, t)
+	assertSeqContentsSet(s1, ints1, t)
+	assertValue(ok, true, t)
+
+	s2, ok := s1.SetVal(5)
+	assertSeqContentsSet(s1, ints1, t)
+	assertSeqContentsSet(s2, ints1, t)
+	assertValue(ok, false, t)
+}
+
+// Test deleting a value from a Set
+func TestDelVal(t *T) {
+	ints := []types.Elem{0, 1, 2, 3, 4}
+	ints1 := []types.Elem{0, 1, 2, 3}
+	ints2 := []types.Elem{1, 2, 3, 4}
+	ints3 := []types.Elem{1, 2, 3, 4, 5}
+
+	// Degenerate case
+	s := NewSet()
+	assertEmpty(s, t)
+	s, ok := s.DelVal(0)
+	assertEmpty(s, t)
+	assertValue(ok, false, t)
+
+	s = NewSet(ints...)
+	s1, ok := s.DelVal(4)
+	assertSeqContentsSet(s, ints, t)
+	assertSeqContentsSet(s1, ints1, t)
+	assertValue(ok, true, t)
+
+	s1, ok = s1.DelVal(4)
+	assertSeqContentsSet(s1, ints1, t)
+	assertValue(ok, false, t)
+
+	// 0 is the value on the root node of s, which is kind of a special case. We
+	// want to test deleting it and setting a new value (which should get put on
+	// the root node).
+	s2, ok := s.DelVal(0)
+	assertSeqContentsSet(s, ints, t)
+	assertSeqContentsSet(s2, ints2, t)
+	assertValue(ok, true, t)
+
+	s2, ok = s2.DelVal(0)
+	assertSeqContentsSet(s2, ints2, t)
+	assertValue(ok, false, t)
+
+	s3, ok := s2.SetVal(5)
+	assertSeqContentsSet(s2, ints2, t)
+	assertSeqContentsSet(s3, ints3, t)
+	assertValue(ok, true, t)
+}
+
+// Test getting values from a Set
+func GetVal(t *T) {
+	//Degenerate case
+	s := NewSet()
+	v, ok := s.GetVal(1)
+	assertValue(v, nil, t)
+	assertValue(ok, false, t)
+
+	s = NewSet(0, 1, 2, 3, 4)
+	v, ok = s.GetVal(1)
+	assertValue(v, 1, t)
+	assertValue(ok, true, t)
+
+	// After delete
+	s, _ = s.DelVal(1)
+	v, ok = s.GetVal(1)
+	assertValue(v, nil, t)
+	assertValue(ok, false, t)
+
+	// After set
+	s, _ = s.SetVal(1)
+	v, ok = s.GetVal(1)
+	assertValue(v, 1, t)
+	assertValue(ok, true, t)
+
+	// After delete root node
+	s, _ = s.DelVal(0)
+	v, ok = s.GetVal(0)
+	assertValue(v, nil, t)
+	assertValue(ok, false, t)
+
+	// After set root node
+	s, _ = s.SetVal(5)
+	v, ok = s.GetVal(5)
+	assertValue(v, 5, t)
+	assertValue(ok, true, t)
+}
+
+// Test that Size functions properly for all cases
+func TestSetSize(t *T) {
+	// Degenerate case
+	s := NewSet()
+	assertValue(s.Size(), uint64(0), t)
+
+	// Initialization case
+	s = NewSet(0, 1, 2)
+	assertValue(s.Size(), uint64(3), t)
+
+	// Setting (both value not in and a value already in)
+	s, _ = s.SetVal(3)
+	assertValue(s.Size(), uint64(4), t)
+	s, _ = s.SetVal(3)
+	assertValue(s.Size(), uint64(4), t)
+
+	// Deleting (both value already in and a value not in)
+	s, _ = s.DelVal(3)
+	assertValue(s.Size(), uint64(3), t)
+	s, _ = s.DelVal(3)
+	assertValue(s.Size(), uint64(3), t)
+
+	// Deleting and setting the root node
+	s, _ = s.DelVal(0)
+	assertValue(s.Size(), uint64(2), t)
+	s, _ = s.SetVal(5)
+	assertValue(s.Size(), uint64(3), t)
+
+}
+
+// Test that Union functions properly
+func TestUnion(t *T) {
+	// Degenerate case
+	empty := NewSet()
+	assertEmpty(empty.Union(empty), t)
+
+	ints1 := []types.Elem{0, 1, 2}
+	ints2 := []types.Elem{3, 4, 5}
+	intsu := append(ints1, ints2...)
+	s1 := NewSet(ints1...)
+	s2 := NewSet(ints2...)
+
+	assertSeqContentsSet(s1.Union(empty), ints1, t)
+	assertSeqContentsSet(empty.Union(s1), ints1, t)
+
+	su := s1.Union(s2)
+	assertSeqContentsSet(s1, ints1, t)
+	assertSeqContentsSet(s2, ints2, t)
+	assertSeqContentsSet(su, intsu, t)
+}
+
+// Test that Intersection functions properly
+func TestIntersection(t *T) {
+	// Degenerate case
+	empty := NewSet()
+	assertEmpty(empty.Intersection(empty), t)
+
+	ints1 := []types.Elem{0, 1, 2}
+	ints2 := []types.Elem{1, 2, 3}
+	ints3 := []types.Elem{4, 5, 6}
+	intsi := []types.Elem{1, 2}
+	s1 := NewSet(ints1...)
+	s2 := NewSet(ints2...)
+	s3 := NewSet(ints3...)
+
+	assertEmpty(s1.Intersection(empty), t)
+	assertEmpty(empty.Intersection(s1), t)
+
+	si := s1.Intersection(s2)
+	assertEmpty(s1.Intersection(s3), t)
+	assertSeqContentsSet(s1, ints1, t)
+	assertSeqContentsSet(s2, ints2, t)
+	assertSeqContentsSet(s3, ints3, t)
+	assertSeqContentsSet(si, intsi, t)
+}
+
+// Test that Difference functions properly
+func TestDifference(t *T) {
+	// Degenerate case
+	empty := NewSet()
+	assertEmpty(empty.Difference(empty), t)
+
+	ints1 := []types.Elem{0, 1, 2, 3}
+	ints2 := []types.Elem{2, 3, 4}
+	intsd := []types.Elem{0, 1}
+	s1 := NewSet(ints1...)
+	s2 := NewSet(ints2...)
+
+	assertSeqContentsSet(s1.Difference(empty), ints1, t)
+	assertEmpty(empty.Difference(s1), t)
+
+	sd := s1.Difference(s2)
+	assertSeqContentsSet(s1, ints1, t)
+	assertSeqContentsSet(s2, ints2, t)
+	assertSeqContentsSet(sd, intsd, t)
+}
+
+// Test that SymDifference functions properly
+func TestSymDifference(t *T) {
+	// Degenerate case
+	empty := NewSet()
+	assertEmpty(empty.SymDifference(empty), t)
+
+	ints1 := []types.Elem{0, 1, 2, 3}
+	ints2 := []types.Elem{2, 3, 4}
+	intsd := []types.Elem{0, 1, 4}
+	s1 := NewSet(ints1...)
+	s2 := NewSet(ints2...)
+
+	assertSeqContentsSet(s1.SymDifference(empty), ints1, t)
+	assertSeqContentsSet(empty.SymDifference(s1), ints1, t)
+
+	sd := s1.SymDifference(s2)
+	assertSeqContentsSet(s1, ints1, t)
+	assertSeqContentsSet(s2, ints2, t)
+	assertSeqContentsSet(sd, intsd, t)
+}
diff --git a/seq/lazy.go b/seq/lazy.go
new file mode 100644
index 0000000..97aa1ec
--- /dev/null
+++ b/seq/lazy.go
@@ -0,0 +1,146 @@
+package seq
+
+import (
+	"github.com/mediocregopher/ginger/types"
+)
+
+// A Lazy is an implementation of a Seq which only actually evaluates its
+// contents as those contents become needed. Lazys can be chained together, so
+// if you have three steps in a pipeline there aren't two intermediate Seqs
+// created, only the final resulting one. Lazys are also thread-safe, so
+// multiple routines can interact with the same Lazy pointer at the same time
+// but the contents will only be evalutated once.
+type Lazy struct {
+	this types.Elem
+	next *Lazy
+	ok   bool
+	ch   chan struct{}
+}
+
+// Given a Thunk, returns a Lazy around that Thunk.
+func NewLazy(t Thunk) *Lazy {
+	l := &Lazy{ch: make(chan struct{})}
+	go func() {
+		l.ch <- struct{}{}
+		el, next, ok := t()
+		l.this = el
+		l.next = NewLazy(next)
+		l.ok = ok
+		close(l.ch)
+	}()
+	return l
+}
+
+// Implementation of FirstRest for Seq interface. Completes in O(1) time.
+func (l *Lazy) FirstRest() (types.Elem, Seq, bool) {
+	if l == nil {
+		return nil, l, false
+	}
+
+	// Reading from the channel tells the Lazy to populate the data and prepare
+	// the next item in the seq, it closes the channel when it's done that.
+	if _, ok := <-l.ch; ok {
+		<-l.ch
+	}
+
+	if l.ok {
+		return l.this, l.next, true
+	} else {
+		return nil, nil, false
+	}
+}
+
+// Implementation of String for Stringer
+func (l *Lazy) String() string {
+	return ToString(l, "<<", ">>")
+}
+
+// Thunks are the building blocks a Lazy. A Thunk returns an element, another
+// Thunk, and a boolean representing if the call yielded any results or if it
+// was actually empty (true indicates it yielded results).
+type Thunk func() (types.Elem, Thunk, bool)
+
+func mapThunk(fn func(types.Elem) types.Elem, s Seq) Thunk {
+	return func() (types.Elem, Thunk, bool) {
+		el, ns, ok := s.FirstRest()
+		if !ok {
+			return nil, nil, false
+		}
+
+		return fn(el), mapThunk(fn, ns), true
+	}
+}
+
+// Lazy implementation of Map
+func LMap(fn func(types.Elem) types.Elem, s Seq) Seq {
+	return NewLazy(mapThunk(fn, s))
+}
+
+func filterThunk(fn func(types.Elem) bool, s Seq) Thunk {
+	return func() (types.Elem, Thunk, bool) {
+		for {
+			el, ns, ok := s.FirstRest()
+			if !ok {
+				return nil, nil, false
+			}
+
+			if keep := fn(el); keep {
+				return el, filterThunk(fn, ns), true
+			} else {
+				s = ns
+			}
+		}
+	}
+}
+
+// Lazy implementation of Filter
+func LFilter(fn func(types.Elem) bool, s Seq) Seq {
+	return NewLazy(filterThunk(fn, s))
+}
+
+func takeThunk(n uint64, s Seq) Thunk {
+	return func() (types.Elem, Thunk, bool) {
+		el, ns, ok := s.FirstRest()
+		if !ok || n == 0 {
+			return nil, nil, false
+		}
+		return el, takeThunk(n-1, ns), true
+	}
+}
+
+// Lazy implementation of Take
+func LTake(n uint64, s Seq) Seq {
+	return NewLazy(takeThunk(n, s))
+}
+
+func takeWhileThunk(fn func(types.Elem) bool, s Seq) Thunk {
+	return func() (types.Elem, Thunk, bool) {
+		el, ns, ok := s.FirstRest()
+		if !ok || !fn(el) {
+			return nil, nil, false
+		}
+		return el, takeWhileThunk(fn, ns), true
+	}
+}
+
+// Lazy implementation of TakeWhile
+func LTakeWhile(fn func(types.Elem) bool, s Seq) Seq {
+	return NewLazy(takeWhileThunk(fn, s))
+}
+
+func toLazyThunk(s Seq) Thunk {
+	return func() (types.Elem, Thunk, bool) {
+		el, ns, ok := s.FirstRest()
+		if !ok {
+			return nil, nil, false
+		}
+		return el, toLazyThunk(ns), true
+	}
+}
+
+// Returns the Seq as a Lazy. Pointless for linked-lists, but possibly useful
+// for other implementations where FirstRest might be costly and the same Seq
+// needs to be iterated over many times.
+func ToLazy(s Seq) *Lazy {
+	return NewLazy(toLazyThunk(s))
+}
diff --git a/seq/lazy_test.go b/seq/lazy_test.go
new file mode 100644
index 0000000..98cc1a9
--- /dev/null
+++ b/seq/lazy_test.go
@@ -0,0 +1,49 @@
+package seq
+
+import (
+	. "testing"
+	"time"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Test lazy operation and thread-safety
+func TestLazyBasic(t *T) {
+	ch := make(chan int)
+	mapfn := func(el types.Elem) types.Elem {
+		i := el.(int)
+		ch <- i
+		return i
+	}
+
+	intl := []types.Elem{0, 1, 2, 3, 4}
+	l := NewList(intl...)
+	ml := LMap(mapfn, l)
+
+	for i := 0; i < 10; i++ {
+		go func() {
+			mlintl := ToSlice(ml)
+			if !intSlicesEq(mlintl, intl) {
+				panic("contents not right")
+			}
+		}()
+	}
+
+	for _, el := range intl {
+		select {
+		case elch := <-ch:
+			assertValue(el, elch, t)
+		case <-time.After(1 * time.Millisecond):
+			t.Fatalf("Took too long reading result")
+		}
+	}
+	close(ch)
+}
+
+// Test that arbitrary Seqs can turn into Lazy
+func TestToLazy(t *T) {
+	intl := []types.Elem{0, 1, 2, 3, 4}
+	l := NewList(intl...)
+	ll := ToLazy(l)
+	assertSeqContents(ll, intl, t)
+}
diff --git a/seq/list.go b/seq/list.go
new file mode 100644
index 0000000..86c71db
--- /dev/null
+++ b/seq/list.go
@@ -0,0 +1,142 @@
+package seq
+
+import (
+	"github.com/mediocregopher/ginger/types"
+)
+
+// A List is an implementation of Seq in the form of a single-linked-list, and
+// is used as the underlying structure for Seqs for most methods that return a
+// Seq. It is probably the most efficient and simplest of the implementations.
+// Even though, conceptually, all Seq operations return a new Seq, the old Seq
+// can actually share nodes with the new Seq (if both are Lists), thereby saving
+// memory and copies.
+type List struct {
+	el   types.Elem
+	next *List
+}
+
+// Returns a new List comprised of the given elements (or no elements, for an
+// empty list)
+func NewList(els ...types.Elem) *List {
+	elsl := len(els)
+	if elsl == 0 {
+		return nil
+	}
+
+	var cur *List
+	for i := 0; i < elsl; i++ {
+		cur = &List{els[elsl-i-1], cur}
+	}
+	return cur
+}
+
+// Implementation of FirstRest for Seq interface. Completes in O(1) time.
+func (l *List) FirstRest() (types.Elem, Seq, bool) {
+	if l == nil {
+		return nil, l, false
+	} else {
+		return l.el, l.next, true
+	}
+}
+
+// Implementation of String for Stringer interface.
+func (l *List) String() string {
+	return ToString(l, "(", ")")
+}
+
+// Prepends the given element to the front of the list, returning a copy of the
+// new list. Completes in O(1) time.
+func (l *List) Prepend(el types.Elem) *List {
+	return &List{el, l}
+}
+
+// Prepends the argument Seq to the beginning of the callee List, returning a
+// copy of the new List. Completes in O(N) time, N being the length of the
+// argument Seq
+func (l *List) PrependSeq(s Seq) *List {
+	var first, cur, prev *List
+	var el types.Elem
+	var ok bool
+	for {
+		el, s, ok = s.FirstRest()
+		if !ok {
+			break
+		}
+		cur = &List{el, nil}
+		if first == nil {
+			first = cur
+		}
+		if prev != nil {
+			prev.next = cur
+		}
+		prev = cur
+	}
+
+	// prev will be nil if s is empty
+	if prev == nil {
+		return l
+	}
+
+	prev.next = l
+	return first
+}
+
+// Appends the given element to the end of the List, returning a copy of the new
+// List. While most methods on List don't actually copy much data, this one
+// copies the entire list. Completes in O(N) time.
+func (l *List) Append(el types.Elem) *List {
+	var first, cur, prev *List
+	for l != nil {
+		cur = &List{l.el, nil}
+		if first == nil {
+			first = cur
+		}
+		if prev != nil {
+			prev.next = cur
+		}
+		prev = cur
+		l = l.next
+	}
+	final := &List{el, nil}
+	if prev == nil {
+		return final
+	}
+	prev.next = final
+	return first
+}
+
+// Returns the nth index element (starting at 0), with bool being false if i is
+// out of bounds. Completes in O(N) time.
+func (l *List) Nth(n uint64) (types.Elem, bool) {
+	var el types.Elem
+	var ok bool
+	s := Seq(l)
+	for i := uint64(0); ; i++ {
+		el, s, ok = s.FirstRest()
+		if !ok {
+			return nil, false
+		} else if i == n {
+			return el, true
+		}
+	}
+}
+
+// Returns the elements in the Seq as a List. Has similar properties as
+// ToSlice. In general this completes in O(N) time. If the given Seq is already
+// a List it will complete in O(1) time.
+func ToList(s Seq) *List {
+	var ok bool
+	var l *List
+	if l, ok = s.(*List); ok {
+		return l
+	}
+
+	var el types.Elem
+	for ret := NewList(); ; {
+		if el, s, ok = s.FirstRest(); ok {
+			ret = ret.Prepend(el)
+		} else {
+			return Reverse(ret).(*List)
+		}
+	}
+}
diff --git a/seq/list_test.go b/seq/list_test.go
new file mode 100644
index 0000000..05796cf
--- /dev/null
+++ b/seq/list_test.go
@@ -0,0 +1,156 @@
+package seq
+
+import (
+	. "testing"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Asserts that the given list is properly formed and has all of its size fields
+// filled in correctly
+func assertSaneList(l *List, t *T) {
+	if Size(l) == 0 {
+		var nilpointer *List
+		assertValue(l, nilpointer, t)
+		return
+	}
+
+	size := Size(l)
+	assertValue(Size(l.next), size-1, t)
+	assertSaneList(l.next, t)
+}
+
+// Test creating a list and calling the Seq interface methods on it
+func TestListSeq(t *T) {
+	ints := []types.Elem{1, "a", 5.0}
+
+	// Testing creation and Seq interface methods
+	l := NewList(ints...)
+	sl := testSeqGen(t, l, ints)
+
+	// sl should be empty at this point
+	l = ToList(sl)
+	var nilpointer *List
+	assertEmpty(l, t)
+	assertValue(l, nilpointer, t)
+	assertValue(len(ToSlice(l)), 0, t)
+
+	// Testing creation of empty List.
+	emptyl := NewList()
+	assertValue(emptyl, nilpointer, t)
+}
+
+// Test the string representation of a List
+func TestStringSeq(t *T) {
+	l := NewList(0, 1, 2, 3)
+	assertValue(l.String(), "( 0 1 2 3 )", t)
+
+	l = NewList(0, 1, 2, NewList(3, 4), 5, NewList(6, 7, 8))
+	assertValue(l.String(), "( 0 1 2 ( 3 4 ) 5 ( 6 7 8 ) )", t)
+}
+
+// Test prepending an element to the beginning of a list
+func TestPrepend(t *T) {
+	// Normal case
+	intl := []types.Elem{3, 2, 1, 0}
+	l := NewList(intl...)
+	nl := l.Prepend(4)
+	assertSaneList(l, t)
+	assertSaneList(nl, t)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{4, 3, 2, 1, 0}, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = l.Prepend(0)
+	assertEmpty(l, t)
+	assertSaneList(nl, t)
+	assertSeqContents(nl, []types.Elem{0}, t)
+}
+
+// Test prepending a Seq to the beginning of a list
+func TestPrependSeq(t *T) {
+	//Normal case
+	intl1 := []types.Elem{3, 4}
+	intl2 := []types.Elem{0, 1, 2}
+	l1 := NewList(intl1...)
+	l2 := NewList(intl2...)
+	nl := l1.PrependSeq(l2)
+	assertSaneList(l1, t)
+	assertSaneList(l2, t)
+	assertSaneList(nl, t)
+	assertSeqContents(l1, intl1, t)
+	assertSeqContents(l2, intl2, t)
+	assertSeqContents(nl, []types.Elem{0, 1, 2, 3, 4}, t)
+
+	// Degenerate cases
+	blank1 := NewList()
+	blank2 := NewList()
+	nl = blank1.PrependSeq(blank2)
+	assertEmpty(blank1, t)
+	assertEmpty(blank2, t)
+	assertEmpty(nl, t)
+
+	nl = blank1.PrependSeq(l1)
+	assertEmpty(blank1, t)
+	assertSaneList(nl, t)
+	assertSeqContents(nl, intl1, t)
+
+	nl = l1.PrependSeq(blank1)
+	assertEmpty(blank1, t)
+	assertSaneList(nl, t)
+	assertSeqContents(nl, intl1, t)
+}
+
+// Test appending to the end of a List
+func TestAppend(t *T) {
+	// Normal case
+	intl := []types.Elem{3, 2, 1}
+	l := NewList(intl...)
+	nl := l.Append(0)
+	assertSaneList(l, t)
+	assertSaneList(nl, t)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{3, 2, 1, 0}, t)
+
+	// Edge case (algorithm gets weird here)
+	l = NewList(1)
+	nl = l.Append(0)
+	assertSaneList(l, t)
+	assertSaneList(nl, t)
+	assertSeqContents(l, []types.Elem{1}, t)
+	assertSeqContents(nl, []types.Elem{1, 0}, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = l.Append(0)
+	assertEmpty(l, t)
+	assertSaneList(nl, t)
+	assertSeqContents(nl, []types.Elem{0}, t)
+}
+
+// Test retrieving items from a List
+func TestNth(t *T) {
+	// Normal case, in bounds
+	intl := []types.Elem{0, 2, 4, 6, 8}
+	l := NewList(intl...)
+	r, ok := l.Nth(3)
+	assertSaneList(l, t)
+	assertSeqContents(l, intl, t)
+	assertValue(r, 6, t)
+	assertValue(ok, true, t)
+
+	// Normal case, out of bounds
+	r, ok = l.Nth(8)
+	assertSaneList(l, t)
+	assertSeqContents(l, intl, t)
+	assertValue(r, nil, t)
+	assertValue(ok, false, t)
+
+	// Degenerate case
+	l = NewList()
+	r, ok = l.Nth(0)
+	assertEmpty(l, t)
+	assertValue(r, nil, t)
+	assertValue(ok, false, t)
+}
diff --git a/seq/seq.go b/seq/seq.go
new file mode 100644
index 0000000..562bd1d
--- /dev/null
+++ b/seq/seq.go
@@ -0,0 +1,281 @@
+// This package describes ginger datastructures, and many of the operations
+// which can be used on those data structures
+package seq
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// The general interface which most operations will actually operate on. Acts as
+// an interface onto any data structure
+type Seq interface {
+
+	// Returns the "first" element in the data structure as well as a Seq
+	// containing a copy of the rest of the elements in the data structure. The
+	// "first" element can be random for structures which don't have a concept
+	// of order (like Set). Calling FirstRest on an empty Seq (Size() == 0) will
+	// return "first" as nil, the same empty Seq , and false. The third return
+	// value is true in all other cases.
+	FirstRest() (types.Elem, Seq, bool)
+}
+
+// Returns the number of elements contained in the data structure. In general
+// this completes in O(N) time, except for Set and HashMap for which it
+// completes in O(1)
+func Size(s Seq) uint64 {
+	switch st := s.(type) {
+	case *Set:
+		return st.Size()
+	case *HashMap:
+		return st.Size()
+	default:
+	}
+
+	var ok bool
+	for i := uint64(0); ; {
+		if _, s, ok = s.FirstRest(); ok {
+			i++
+		} else {
+			return i
+		}
+	}
+}
+
+// Returns the elements in the Seq as a slice. If the underlying Seq has any
+// implicit order to it that order will be kept. An empty Seq will return an
+// empty slice; nil is never returned. In general this completes in O(N) time.
+func ToSlice(s Seq) []types.Elem {
+	var el types.Elem
+	var ok bool
+	for ret := make([]types.Elem, 0, 8); ; {
+		if el, s, ok = s.FirstRest(); ok {
+			ret = append(ret, el)
+		} else {
+			return ret
+		}
+	}
+}
+
+// Turns a Seq into a string, with each element separated by a space and with a
+// dstart and dend wrapping the whole thing
+func ToString(s Seq, dstart, dend string) string {
+	buf := bytes.NewBufferString(dstart)
+	buf.WriteString(" ")
+	var el types.Elem
+	var strel fmt.Stringer
+	var rest Seq
+	var ok bool
+	for {
+		if el, rest, ok = s.FirstRest(); ok {
+			if strel, ok = el.(fmt.Stringer); ok {
+				buf.WriteString(strel.String())
+			} else {
+				buf.WriteString(fmt.Sprintf("%v", el))
+			}
+			buf.WriteString(" ")
+			s = rest
+		} else {
+			break
+		}
+	}
+	buf.WriteString(dend)
+	return buf.String()
+}
+
+// Returns a reversed copy of the List. Completes in O(N) time.
+func Reverse(s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			l = l.Prepend(el)
+		} else {
+			return l
+		}
+	}
+}
+
+// Returns a Seq consisting of the result of applying fn to each element in the
+// given Seq. Completes in O(N) time.
+func Map(fn func(types.Elem) types.Elem, s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			l = l.Prepend(fn(el))
+		} else {
+			break
+		}
+	}
+	return Reverse(l)
+}
+
+// A function used in a reduce. The first argument is the accumulator, the
+// second is an element from the Seq being reduced over. The ReduceFn returns
+// the accumulator to be used in the next iteration, wherein that new
+// accumulator will be called alongside the next element in the Seq. ReduceFn
+// also returns a boolean representing whether or not the reduction should stop
+// at this step. If true, the reductions will stop and any remaining elements in
+// the Seq will be ignored.
+type ReduceFn func(acc, el types.Elem) (types.Elem, bool)
+
+// Reduces over the given Seq using ReduceFn, with acc as the first accumulator
+// value in the reduce. See ReduceFn for more details on how it works. The
+// return value is the result of the reduction. Completes in O(N) time.
+func Reduce(fn ReduceFn, acc types.Elem, s Seq) types.Elem {
+	var el types.Elem
+	var ok, stop bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			acc, stop = fn(acc, el)
+			if stop {
+				break
+			}
+		} else {
+			break
+		}
+	}
+	return acc
+}
+
+// Returns the first element in Seq for which fn returns true, or nil. The
+// returned boolean indicates whether or not a matching element was found.
+// Completes in O(N) time.
+func Any(fn func(el types.Elem) bool, s Seq) (types.Elem, bool) {
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			if fn(el) {
+				return el, true
+			}
+		} else {
+			return nil, false
+		}
+	}
+}
+
+// Returns true if fn returns true for all elements in the Seq. Completes in
+// O(N) time.
+func All(fn func(types.Elem) bool, s Seq) bool {
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			if !fn(el) {
+				return false
+			}
+		} else {
+			return true
+		}
+	}
+}
+
+// Returns a Seq containing all elements in the given Seq for which fn returned
+// true. Completes in O(N) time.
+func Filter(fn func(el types.Elem) bool, s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			if fn(el) {
+				l = l.Prepend(el)
+			}
+		} else {
+			return Reverse(l)
+		}
+	}
+}
+
+// Flattens the given Seq into a single, one-dimensional Seq. This method only
+// flattens Seqs found in the top level of the given Seq, it does not recurse
+// down to multiple layers. Completes in O(N*M) time, where N is the number of
+// elements in the Seq and M is how large the Seqs in those elements actually
+// are.
+func Flatten(s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for {
+		if el, s, ok = s.FirstRest(); ok {
+			if els, ok := el.(Seq); ok {
+				l = l.PrependSeq(Reverse(els))
+			} else {
+				l = l.Prepend(el)
+			}
+		} else {
+			return Reverse(l)
+		}
+	}
+}
+
+// Returns a Seq containing the first n elements in the given Seq. If n is
+// greater than the length of the given Seq then the whole Seq is returned.
+// Completes in O(N) time.
+func Take(n uint64, s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for i := uint64(0); i < n; i++ {
+		el, s, ok = s.FirstRest()
+		if !ok {
+			break
+		}
+		l = l.Prepend(el)
+	}
+	return Reverse(l)
+}
+
+// Goes through each item in the given Seq until an element returns false from
+// pred. Returns a new Seq containing these truthful elements. Completes in O(N)
+// time.
+func TakeWhile(pred func(types.Elem) bool, s Seq) Seq {
+	l := NewList()
+	var el types.Elem
+	var ok bool
+	for {
+		el, s, ok = s.FirstRest()
+		if !ok || !pred(el) {
+			break
+		}
+		l = l.Prepend(el)
+	}
+	return Reverse(l)
+}
+
+// Returns a Seq the is the previous Seq without the first n elements. If n is
+// greater than the length of the Seq, returns an empty Seq. Completes in O(N)
+// time.
+func Drop(n uint64, s Seq) Seq {
+	var ok bool
+	for i := uint64(0); i < n; i++ {
+		_, s, ok = s.FirstRest()
+		if !ok {
+			break
+		}
+	}
+	return s
+}
+
+// Drops elements from the given Seq until pred returns false for an element.
+// Returns a Seq of the remaining elements (including the one which returned
+// false). Completes in O(N) time.
+func DropWhile(pred func(types.Elem) bool, s Seq) Seq {
+	var el types.Elem
+	var curs Seq
+	var ok bool
+	for {
+		el, curs, ok = s.FirstRest()
+		if !ok || !pred(el) {
+			break
+		}
+		s = curs
+	}
+	return s
+}
diff --git a/seq/seq_test.go b/seq/seq_test.go
new file mode 100644
index 0000000..991e8c9
--- /dev/null
+++ b/seq/seq_test.go
@@ -0,0 +1,372 @@
+package seq
+
+import (
+	. "testing"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Tests the FirstRest, Size, and ToSlice methods of a Seq
+func testSeqGen(t *T, s Seq, ints []types.Elem) Seq {
+	intsl := uint64(len(ints))
+	for i := range ints {
+		assertSaneList(ToList(s), t)
+		assertValue(Size(s), intsl-uint64(i), t)
+		assertSeqContents(s, ints[i:], t)
+
+		first, rest, ok := s.FirstRest()
+		assertValue(ok, true, t)
+		assertValue(first, ints[i], t)
+
+		s = rest
+	}
+	return s
+}
+
+// Tests the FirstRest, Size, and ToSlice methods of an unordered Seq
+func testSeqNoOrderGen(t *T, s Seq, ints []types.Elem) Seq {
+	intsl := uint64(len(ints))
+
+	m := map[types.Elem]bool{}
+	for i := range ints {
+		m[ints[i]] = true
+	}
+
+	for i := range ints {
+		assertSaneList(ToList(s), t)
+		assertValue(Size(s), intsl-uint64(i), t)
+		assertSeqContentsNoOrderMap(s, m, t)
+
+		first, rest, ok := s.FirstRest()
+		assertValue(ok, true, t)
+		assertInMap(first, m, t)
+
+		delete(m, first)
+		s = rest
+	}
+	return s
+}
+
+// Test reversing a Seq
+func TestReverse(t *T) {
+	// Normal case
+	intl := []types.Elem{3, 2, 1}
+	l := NewList(intl...)
+	nl := Reverse(l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{1, 2, 3}, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = Reverse(l)
+	assertEmpty(l, t)
+	assertEmpty(nl, t)
+}
+
+func testMapGen(t *T, mapFn func(func(types.Elem) types.Elem, Seq) Seq) {
+	fn := func(n types.Elem) types.Elem {
+		return n.(int) + 1
+	}
+
+	// Normal case
+	intl := []types.Elem{1, 2, 3}
+	l := NewList(intl...)
+	nl := mapFn(fn, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{2, 3, 4}, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = mapFn(fn, l)
+	assertEmpty(l, t)
+	assertEmpty(nl, t)
+}
+
+// Test mapping over a Seq
+func TestMap(t *T) {
+	testMapGen(t, Map)
+}
+
+// Test lazily mapping over a Seq
+func TestLMap(t *T) {
+	testMapGen(t, LMap)
+}
+
+// Test reducing over a Seq
+func TestReduce(t *T) {
+	fn := func(acc, el types.Elem) (types.Elem, bool) {
+		return acc.(int) + el.(int), false
+	}
+
+	// Normal case
+	intl := []types.Elem{1, 2, 3, 4}
+	l := NewList(intl...)
+	r := Reduce(fn, 0, l)
+	assertSeqContents(l, intl, t)
+	assertValue(r, 10, t)
+
+	// Short-circuit case
+	fns := func(acc, el types.Elem) (types.Elem, bool) {
+		return acc.(int) + el.(int), el.(int) > 2
+	}
+	r = Reduce(fns, 0, l)
+	assertSeqContents(l, intl, t)
+	assertValue(r, 6, t)
+
+	// Degenerate case
+	l = NewList()
+	r = Reduce(fn, 0, l)
+	assertEmpty(l, t)
+	assertValue(r, 0, t)
+}
+
+// Test the Any function
+func TestAny(t *T) {
+	fn := func(el types.Elem) bool {
+		return el.(int) > 3
+	}
+
+	// Value found case
+	intl := []types.Elem{1, 2, 3, 4}
+	l := NewList(intl...)
+	r, ok := Any(fn, l)
+	assertSeqContents(l, intl, t)
+	assertValue(r, 4, t)
+	assertValue(ok, true, t)
+
+	// Value not found case
+	intl = []types.Elem{1, 2, 3}
+	l = NewList(intl...)
+	r, ok = Any(fn, l)
+	assertSeqContents(l, intl, t)
+	assertValue(r, nil, t)
+	assertValue(ok, false, t)
+
+	// Degenerate case
+	l = NewList()
+	r, ok = Any(fn, l)
+	assertEmpty(l, t)
+	assertValue(r, nil, t)
+	assertValue(ok, false, t)
+}
+
+// Test the All function
+func TestAll(t *T) {
+	fn := func(el types.Elem) bool {
+		return el.(int) > 3
+	}
+
+	// All match case
+	intl := []types.Elem{4, 5, 6}
+	l := NewList(intl...)
+	ok := All(fn, l)
+	assertSeqContents(l, intl, t)
+	assertValue(ok, true, t)
+
+	// Not all match case
+	intl = []types.Elem{3, 4, 2, 5}
+	l = NewList(intl...)
+	ok = All(fn, l)
+	assertSeqContents(l, intl, t)
+	assertValue(ok, false, t)
+
+	// Degenerate case
+	l = NewList()
+	ok = All(fn, l)
+	assertEmpty(l, t)
+	assertValue(ok, true, t)
+}
+
+func testFilterGen(t *T, filterFn func(func(types.Elem) bool, Seq) Seq) {
+	fn := func(el types.Elem) bool {
+		return el.(int)%2 != 0
+	}
+
+	// Normal case
+	intl := []types.Elem{1, 2, 3, 4, 5}
+	l := NewList(intl...)
+	r := filterFn(fn, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(r, []types.Elem{1, 3, 5}, t)
+
+	// Degenerate cases
+	l = NewList()
+	r = filterFn(fn, l)
+	assertEmpty(l, t)
+	assertEmpty(r, t)
+}
+
+// Test the Filter function
+func TestFilter(t *T) {
+	testFilterGen(t, Filter)
+}
+
+// Test the lazy Filter function
+func TestLFilter(t *T) {
+	testFilterGen(t, LFilter)
+}
+
+// Test Flatten-ing of a Seq
+func TestFlatten(t *T) {
+	// Normal case
+	intl1 := []types.Elem{0, 1, 2}
+	intl2 := []types.Elem{3, 4, 5}
+	l1 := NewList(intl1...)
+	l2 := NewList(intl2...)
+	blank := NewList()
+	intl := []types.Elem{-1, l1, l2, 6, blank, 7}
+	l := NewList(intl...)
+	nl := Flatten(l)
+	assertSeqContents(l1, intl1, t)
+	assertSeqContents(l2, intl2, t)
+	assertEmpty(blank, t)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{-1, 0, 1, 2, 3, 4, 5, 6, 7}, t)
+
+	// Degenerate case
+	nl = Flatten(blank)
+	assertEmpty(blank, t)
+	assertEmpty(nl, t)
+}
+
+func testTakeGen(t *T, takeFn func(uint64, Seq) Seq) {
+	// Normal case
+	intl := []types.Elem{0, 1, 2, 3, 4}
+	l := NewList(intl...)
+	nl := takeFn(3, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{0, 1, 2}, t)
+
+	// Edge cases
+	nl = takeFn(5, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, intl, t)
+
+	nl = takeFn(6, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, intl, t)
+
+	// Degenerate cases
+	empty := NewList()
+	nl = takeFn(1, empty)
+	assertEmpty(empty, t)
+	assertEmpty(nl, t)
+
+	nl = takeFn(0, l)
+	assertSeqContents(l, intl, t)
+	assertEmpty(nl, t)
+}
+
+// Test taking from a Seq
+func TestTake(t *T) {
+	testTakeGen(t, Take)
+}
+
+// Test lazily taking from a Seq
+func TestLTake(t *T) {
+	testTakeGen(t, LTake)
+}
+
+func testTakeWhileGen(t *T, takeWhileFn func(func(types.Elem) bool, Seq) Seq) {
+	pred := func(el types.Elem) bool {
+		return el.(int) < 3
+	}
+
+	// Normal case
+	intl := []types.Elem{0, 1, 2, 3, 4, 5}
+	l := NewList(intl...)
+	nl := takeWhileFn(pred, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{0, 1, 2}, t)
+
+	// Edge cases
+	intl = []types.Elem{5, 5, 5}
+	l = NewList(intl...)
+	nl = takeWhileFn(pred, l)
+	assertSeqContents(l, intl, t)
+	assertEmpty(nl, t)
+
+	intl = []types.Elem{0, 1, 2}
+	l = NewList(intl...)
+	nl = takeWhileFn(pred, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{0, 1, 2}, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = takeWhileFn(pred, l)
+	assertEmpty(l, t)
+	assertEmpty(nl, t)
+}
+
+// Test taking from a Seq until a given condition
+func TestTakeWhile(t *T) {
+	testTakeWhileGen(t, TakeWhile)
+}
+
+// Test lazily taking from a Seq until a given condition
+func TestLTakeWhile(t *T) {
+	testTakeWhileGen(t, LTakeWhile)
+}
+
+// Test dropping from a Seq
+func TestDrop(t *T) {
+	// Normal case
+	intl := []types.Elem{0, 1, 2, 3, 4}
+	l := NewList(intl...)
+	nl := Drop(3, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{3, 4}, t)
+
+	// Edge cases
+	nl = Drop(5, l)
+	assertSeqContents(l, intl, t)
+	assertEmpty(nl, t)
+
+	nl = Drop(6, l)
+	assertSeqContents(l, intl, t)
+	assertEmpty(nl, t)
+
+	// Degenerate cases
+	empty := NewList()
+	nl = Drop(1, empty)
+	assertEmpty(empty, t)
+	assertEmpty(nl, t)
+
+	nl = Drop(0, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, intl, t)
+}
+
+// Test dropping from a Seq until a given condition
+func TestDropWhile(t *T) {
+	pred := func(el types.Elem) bool {
+		return el.(int) < 3
+	}
+
+	// Normal case
+	intl := []types.Elem{0, 1, 2, 3, 4, 5}
+	l := NewList(intl...)
+	nl := DropWhile(pred, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, []types.Elem{3, 4, 5}, t)
+
+	// Edge cases
+	intl = []types.Elem{5, 5, 5}
+	l = NewList(intl...)
+	nl = DropWhile(pred, l)
+	assertSeqContents(l, intl, t)
+	assertSeqContents(nl, intl, t)
+
+	intl = []types.Elem{0, 1, 2}
+	l = NewList(intl...)
+	nl = DropWhile(pred, l)
+	assertSeqContents(l, intl, t)
+	assertEmpty(nl, t)
+
+	// Degenerate case
+	l = NewList()
+	nl = DropWhile(pred, l)
+	assertEmpty(l, t)
+	assertEmpty(nl, t)
+}
diff --git a/seq/util.go b/seq/util.go
new file mode 100644
index 0000000..d482e96
--- /dev/null
+++ b/seq/util.go
@@ -0,0 +1,90 @@
+package seq
+
+import (
+	"testing"
+
+	"github.com/mediocregopher/ginger/types"
+)
+
+// Returns whether or not two types.Elem slices contain the same elements
+func intSlicesEq(a, b []types.Elem) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// Asserts that the given Seq is empty (contains no elements)
+func assertEmpty(s Seq, t *testing.T) {
+	if Size(s) != 0 {
+		t.Fatalf("Seq isn't empty: %v", ToSlice(s))
+	}
+}
+
+// Asserts that the given Seq has the given elements
+func assertSeqContents(s Seq, intl []types.Elem, t *testing.T) {
+	if ls := ToSlice(s); !intSlicesEq(ls, intl) {
+		t.Fatalf("Slice contents wrong: %v not %v", ls, intl)
+	}
+}
+
+// Asserts that the given Seq has all elements, and only the elements, in the
+// given map
+func assertSeqContentsNoOrderMap(s Seq, m map[types.Elem]bool, t *testing.T) {
+	ls := ToSlice(s)
+	if len(ls) != len(m) {
+		t.Fatalf("Slice contents wrong: %v not %v", ls, m)
+	}
+	for i := range ls {
+		if _, ok := m[ls[i]]; !ok {
+			t.Fatalf("Slice contents wrong: %v not %v", ls, m)
+		}
+	}
+}
+
+// Asserts that the given Seq has all the elements, and only the elements
+// (duplicates removed), in the given slice, although no necessarily in the
+// order given in the slice
+func assertSeqContentsSet(s Seq, ints []types.Elem, t *testing.T) {
+	m := map[types.Elem]bool{}
+	for i := range ints {
+		m[ints[i]] = true
+	}
+	assertSeqContentsNoOrderMap(s, m, t)
+}
+
+func assertSeqContentsHashMap(s Seq, kvs []*KV, t *testing.T) {
+	m := map[types.Elem]bool{}
+	for i := range kvs {
+		m[*kvs[i]] = true
+	}
+	ls := ToSlice(s)
+	if len(ls) != len(m) {
+		t.Fatalf("Slice contents wrong: %v not %v", ls, m)
+	}
+	for i := range ls {
+		kv := ls[i].(*KV)
+		if _, ok := m[*kv]; !ok {
+			t.Fatalf("Slice contents wrong: %v not %v", ls, m)
+		}
+	}
+}
+
+// Asserts that v1 is the same as v2
+func assertValue(v1, v2 types.Elem, t *testing.T) {
+	if v1 != v2 {
+		t.Fatalf("Value wrong: %v not %v", v1, v2)
+	}
+}
+
+// Asserts that v1 is a key in the given map
+func assertInMap(v1 types.Elem, m map[types.Elem]bool, t *testing.T) {
+	if _, ok := m[v1]; !ok {
+		t.Fatalf("Value not in set: %v not in %v", v1, m)
+	}
+}
diff --git a/types/types.go b/types/types.go
index e47bbfd..c3df1b7 100644
--- a/types/types.go
+++ b/types/types.go
@@ -6,12 +6,12 @@ package types
 type Elem interface {
 }
 
-type String string
+type Str string
 
-type Integer int
+type Int int
 
 type Float float32
 
 type Char rune
 
-type Error error
+type Err error