trav :: (TreeF a (Tree b) -> TreeF b (Tree b)) -> Tree a -> Tree b
trav psi x = cata (inn . psi) x
trav :: (TreeF a (Tree b) -> TreeF b (Tree b)) -> Tree a -> Tree b
trav psi x = cata (inn . psi) x
trav psi x = cata (inn . psi) x