isCompleteTo :: Eq a => Int -> BinaryTree a -> Bool
isCompleteTo d E
  | d == (-1)          = True
  | otherwise          = False
isCompleteTo :: Eq a => Int -> BinaryTree a -> Bool
isCompleteTo d E
  | d == (-1)          = True
  | otherwise          = False
isCompleteTo d E
  | d == (-1)          = True
  | otherwise          = False