code: def to_dict(self):
          
          expansion_strings = []
  
          for expansion in self.expansions:
              expansion_strings.extend(expansion.to_strings())
  
          return {
              self.type: expansion_strings,
          }
docs: Convert the tree node to its dictionary representation.
  
          :return: an expansion dictionary that represents the type and expansions of this tree node.
          :rtype dict[list[union[str, unicode]]]