cons :: c → b → b
map :: (c → c) → b → b
nil :: b
node :: a → b → c
treemap :: (a → a) → c → c
map(F, nil) → nil
map(Z, cons(U, V)) → cons(Z(U), map(Z, V))
treemap(I, node(P, X1)) → node(I(P), map(treemap(I), X1))