0 :: c
add :: a → c → c
cons :: a → b → b
fold :: (a → c → c) → c → b → c
mul :: a → c → c
nil :: b
plus :: c → c → c
prod :: b → c
s :: c → c
sum :: b → c
times :: c → c → c
fold(F, Y, nil) → Y
fold(G, V, cons(W, P)) → G(W, fold(G, V, P))
plus(0, X1) → X1
plus(s(Y1), U1) → s(plus(Y1, U1))
times(0, V1) → 0
times(s(W1), P1) → plus(times(W1, P1), P1)
sum → fold(add, 0)
prod → fold(mul, s(0))