-- Gibbon program: sum a binary tree

data Tree = Leaf Int
          | Node Tree Tree

sumTree :: Tree -> Int
sumTree tr =
  case tr of
    Leaf n   -> n
    Node l r -> sumTree l + sumTree r

gibbon_main =
  let t = Node (Node (Leaf 1) (Leaf 2)) (Node (Leaf 3) (Leaf 4))
      s = sumTree t
  in s