append :: a → a → a
concat :: a → a
cons :: a → a → a
flatten :: a → a
map :: (a → a) → a → a
nil :: a
node :: a → a → a
map(F, nil) → nil
map(Z, cons(U, V)) → cons(Z(U), map(Z, V))
flatten(node(W, P)) → cons(W, concat(map(flatten, P)))
concat(nil) → nil
concat(cons(X1, Y1)) → append(X1, concat(Y1))
append(nil, U1) → U1
append(cons(V1, W1), P1) → cons(V1, append(W1, P1))