input: def dropLeafNodes(head):
      """
      Implement a function that takes a Huffman tree and drops all its leaf nodes.
      >>> dropLeafNodes((1, (2, (4, None, None), (5, None, None)), (3, (6, None, None), (7, None, None))))
      (1, (2, None, None), (3, None, None))
      >>> dropLeafNodes((1, None, None))
      None
      """
      # If node is not None and doesn't have any children
      if head and not head[1] and not head[2]:
          return None
  
      # Don't change anything if head is None
      if not head:
          return None
  
      # Recursively call dropLeafNodes on left and right subtrees
      head = (head[0], dropLeafNodes(head[1]), dropLeafNodes(head[2]))
  
      # Return head of the modified tree
      return head
output:     # If both children are None, return None
      if head[1] is None and head[2] is None:
          return None