input: def dropLeafNodes(head):
      """
      Takes a Huffman tree and drops all its leaf nodes.
      
      >>> from collections import namedtuple
      >>> Node = namedtuple('Node', ['left', 'right'])
      >>> tree = Node(left=Node(left=Node(left=None, right=None), right=Node(left=None, right=None)), right=Node(left=Node(left=None, right=None), right=Node(left=None, right=None)))
      >>> dropLeafNodes(tree)
      Node(left=Node(left=None, right=None), right=Node(left=None, right=None))
      """
output:     if head is None:
          return None
      
      if head.left is None and head.right is None:
          return None
      
      head = head._replace(left=dropLeafNodes(head.left))
      head = head._replace(right=dropLeafNodes(head.right))
      
      return head