0 :: c
1 :: c
add :: c → a → c
cons :: a → b → b
fold :: (c → a → c) → b → c → c
mul :: c → a → c
nil :: b
prod :: b → c
sum :: b → c
fold(F, nil, Y) → Y
fold(G, cons(V, W), P) → fold(G, W, G(P, V))
sum(X1) → fold(add, X1, 0)
fold(mul, Y1, 1) → prod(Y1)