input: def dropLeafNodes(head): 
      """
      Drop all leaf nodes from the given Huffman tree. 
      >>> dropLeafNodes(None)
      None
      >>> dropLeafNodes((1, (2, (4, None, None), (5, None, None)), (3, (6, None, None), (7, None, None))))
      (1, (2, None, None), (3, None, None))
      """
output:     if head is None:
          return None
      
      value, left, right = head
      
      if left is None and right is None:
          return None
      
      left = dropLeafNodes(left)
      right = dropLeafNodes(right)
      
      return (value, left, right)