input: 
  def dropLeafNodes(head):
      """ Takes a Huffman tree and drops all its leaf nodes.
      >>> 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))
      """
  
      # If node is not None and doesn't have any children 
      if head and not head.left and not head.right: 
          return None
    
      # Don't change anything if head is None 
      if not head: 
          return None
    
      # Recursively call dropLeafNodes on left and right subtrees
      head.left = dropLeafNodes(head.left) 
      head.right = dropLeafNodes(head.right) 
    
      # Return head of the modified tree
      return head
  
output:     if head[1] is None and head[2] is None:
          return None
      return (head[0], dropLeafNodes(head[1]), dropLeafNodes(head[2]))