cons :: b → c → c
leaf :: a → b
mapt :: (a → a) → b → b
maptlist :: (a → a) → c → c
nil :: c
node :: c → b
mapt(F, leaf(Y)) → leaf(F(Y))
mapt(G, node(V)) → node(maptlist(G, V))
maptlist(I, nil) → nil
maptlist(J, cons(X1, Y1)) → cons(mapt(J, X1), maptlist(J, Y1))