diff --git a/expr/ctx.go b/expr/ctx.go
new file mode 100644
index 0000000..67592a8
--- /dev/null
+++ b/expr/ctx.go
@@ -0,0 +1,24 @@
+package expr
+
+// Ctx contains all the Macros and Identifiers available. A Ctx is based on the
+// parent it was created from. If the current Ctx doesn't have a particular key
+// being looked up, the parent is called instead, and so on. A consequence of
+// this is that keys in the children take precedence over the parent's
+type Ctx struct {
+	Parent *Ctx
+	Macros map[Macro]func(Expr) (Expr, error)
+}
+
+// GetMacro returns the first instance of the given of the given Macro found. If
+// not found nil is returned.
+func (c *Ctx) GetMacro(m Macro) func(Expr) (Expr, error) {
+	if c.Macros != nil {
+		if fn, ok := c.Macros[m]; ok {
+			return fn
+		}
+	}
+	if c.Parent != nil {
+		return c.Parent.GetMacro(m)
+	}
+	return nil
+}
diff --git a/expr/expr.go b/expr/expr.go
index 4054b9c..fa49081 100644
--- a/expr/expr.go
+++ b/expr/expr.go
@@ -19,7 +19,7 @@ import (
 // information, like the token to which Actual was originally parsed from
 type Actual interface {
 	// Initializes an llvm.Value and returns it.
-	LLVMVal(llvm.Builder) llvm.Value
+	LLVMVal(*Ctx, llvm.Builder) llvm.Value
 }
 
 // equaler is used to compare two expressions. The comparison should not take
@@ -43,9 +43,9 @@ type Expr struct {
 // LLVMVal passes its arguments to the underlying Actual instance. It caches the
 // result, so if this is called multiple times the underlying one is only called
 // the first time.
-func (e Expr) LLVMVal(builder llvm.Builder) llvm.Value {
+func (e Expr) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	if e.val == nil {
-		v := e.Actual.LLVMVal(builder)
+		v := e.Actual.LLVMVal(ctx, builder)
 		e.val = &v
 	}
 	return *e.val
@@ -67,7 +67,7 @@ func (e Expr) equal(e2 Expr) bool {
 type Bool bool
 
 // LLVMVal implements the Actual interface method
-func (b Bool) LLVMVal(builder llvm.Builder) llvm.Value {
+func (b Bool) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	return llvm.Value{}
 }
 
@@ -85,7 +85,7 @@ func (b Bool) equal(e equaler) bool {
 type Int int64
 
 // LLVMVal implements the Actual interface method
-func (i Int) LLVMVal(builder llvm.Builder) llvm.Value {
+func (i Int) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	v := builder.CreateAlloca(llvm.Int64Type(), "")
 	builder.CreateStore(llvm.ConstInt(llvm.Int64Type(), uint64(i), false), v)
 	return v
@@ -105,7 +105,7 @@ func (i Int) equal(e equaler) bool {
 type String string
 
 // LLVMVal implements the Actual interface method
-func (s String) LLVMVal(builder llvm.Builder) llvm.Value {
+func (s String) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	return llvm.Value{}
 }
 
@@ -124,7 +124,7 @@ func (s String) equal(e equaler) bool {
 type Identifier string
 
 // LLVMVal implements the Actual interface method
-func (id Identifier) LLVMVal(builder llvm.Builder) llvm.Value {
+func (id Identifier) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	return llvm.Value{}
 }
 
@@ -149,7 +149,7 @@ func (m Macro) String() string {
 }
 
 // LLVMVal implements the Actual interface method
-func (m Macro) LLVMVal(builder llvm.Builder) llvm.Value {
+func (m Macro) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	panic("Macros have no inherent LLVMVal")
 }
 
@@ -176,7 +176,7 @@ func (tup Tuple) String() string {
 }
 
 // LLVMVal implements the Actual interface method
-func (tup Tuple) LLVMVal(builder llvm.Builder) llvm.Value {
+func (tup Tuple) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	return llvm.Value{}
 }
 
@@ -208,14 +208,15 @@ func (s Statement) String() string {
 }
 
 // LLVMVal implements the Actual interface method
-func (s Statement) LLVMVal(builder llvm.Builder) llvm.Value {
+func (s Statement) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	m, ok := s.To.Actual.(Macro)
 	if !ok {
 		// TODO proper error
 		panic("statement To is not a macro")
 	}
-	fn, ok := macros[m]
-	if !ok {
+
+	fn := ctx.GetMacro(m)
+	if fn == nil {
 		// TODO proper error
 		panic(fmt.Sprintf("unknown macro %q", m))
 	}
@@ -224,7 +225,7 @@ func (s Statement) LLVMVal(builder llvm.Builder) llvm.Value {
 		// TODO proper error
 		panic(err)
 	}
-	return newe.LLVMVal(builder)
+	return newe.LLVMVal(ctx, builder)
 }
 
 func (s Statement) equal(e equaler) bool {
@@ -248,7 +249,7 @@ func (b Block) String() string {
 }
 
 // LLVMVal implements the Actual interface method
-func (b Block) LLVMVal(builder llvm.Builder) llvm.Value {
+func (b Block) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
 	return llvm.Value{}
 }
 
diff --git a/expr/macros.go b/expr/macros.go
index 29b83c6..6ca2563 100644
--- a/expr/macros.go
+++ b/expr/macros.go
@@ -8,25 +8,30 @@ import (
 
 type addActual []Expr
 
-func (aa addActual) LLVMVal(builder llvm.Builder) llvm.Value {
-	a := builder.CreateLoad(aa[0].LLVMVal(builder), "")
+func (aa addActual) LLVMVal(ctx *Ctx, builder llvm.Builder) llvm.Value {
+	a := builder.CreateLoad(aa[0].LLVMVal(ctx, builder), "")
 	for i := range aa[1:] {
-		b := builder.CreateLoad(aa[i+1].LLVMVal(builder), "")
+		b := builder.CreateLoad(aa[i+1].LLVMVal(ctx, builder), "")
 		a = builder.CreateAdd(a, b, "")
 	}
 	return a
 }
 
-var macros = map[Macro]func(Expr) (Expr, error){
-	"add": func(e Expr) (Expr, error) {
-		tup, ok := e.Actual.(Tuple)
-		if !ok {
-			// TODO proper error
-			return Expr{}, errors.New("add only accepts a tuple")
-		}
-		return Expr{
-			Actual: addActual(tup),
-			Token:  e.Token,
-		}, nil
+// RootCtx describes what's available to *all* contexts, and is what all
+// contexts should have as the root parent in the tree
+var RootCtx = &Ctx{
+	Macros: map[Macro]func(Expr) (Expr, error){
+		"add": func(e Expr) (Expr, error) {
+			tup, ok := e.Actual.(Tuple)
+			if !ok {
+				// TODO proper error
+				return Expr{}, errors.New("add only accepts a tuple")
+			}
+			// TODO check that it's a tuple of integers too
+			return Expr{
+				Actual: addActual(tup),
+				Token:  e.Token,
+			}, nil
+		},
 	},
 }
diff --git a/main.go b/main.go
index e33217d..980b069 100644
--- a/main.go
+++ b/main.go
@@ -38,7 +38,7 @@ func main() {
 	addMacro := expr.Expr{Actual: expr.Macro("add")}
 	stmt := expr.Expr{Actual: expr.Statement{In: tup, To: addMacro}}
 
-	result := stmt.LLVMVal(builder)
+	result := stmt.LLVMVal(expr.RootCtx, builder)
 	builder.CreateRet(result)
 
 	// verify it's all good