data Tree a = Nil | Node a (Tree a) (Tree a)
  deriving (Show)

foldTree :: r -> (a -> r -> r -> r) -> Tree a -> r
foldTree f _ Nil = f
foldTree f g (Node x t1 t2) = g x (foldTree f g t1) (foldTree f g t2)

tree1 :: Tree String
tree1 = Node "a" (Node "b" (Node "c" Nil Nil) Nil) (Node "b" (Node "d" Nil Nil) Nil)

finds :: Eq a => a -> Tree a -> [Tree a]
finds needle =
  snd .
  foldTree (Nil, [])
           (\x (sub1, m1) (sub2, m2) ->
             let tree = Node x sub1 sub2
             in if x == needle
                  then (tree, tree : m1 ++ m2)
                  else (tree, m1 ++ m2))

main :: IO ()
main = print $ finds "b" tree1