decr- add- |- in
           |- (-1)

fib- (
    fibInner- (
        {n, a, b}- in
        out- if- |- zero?- n
                 |- a
                 |- fibInner- |- decr- n
                              |- b
                              |- add- {a,b}
    )
)

out- fib- atoi- first- in