data Tree = Leaf Int
          | Fork Tree Int Tree
     deriving Show


t0 = Leaf 0
t1 = Fork (Fork (Leaf 1) 2 (Leaf 3)) 4 (Fork (Leaf 5) 6 (Leaf 7))
t2 = Fork (Fork (Fork (Leaf 1) 2 (Leaf 3)) 4 (Leaf 5)) 6 (Leaf 7)