DATA Tree 
   | Bin   lt: Tree           
           rt: Tree
   | Leaf  x: Int
           it: Item

DATA Item
   | Item
      
SEM Tree [ i: Int | | val: Int ] 
  | Bin   (loc.x, loc.z)  = (@loc.z*2, 2+@loc.y)
          (loc.y,lhs.val) = (0,@lt.val)
          rt.i    = 1
  | Leaf  lhs.val = @lhs.i + @x * @it.val

SEM Item [ | | val: Int ]
  | Item  lhs.val = 2