code: def serialize(self):
          
          res = '<?xml version="1.0" encoding="UTF-8"?>'
          for ns in self.namespaces:
              self.top_grammar.attr["xmlns:" + self.namespaces[ns]] = ns
          res += self.top_grammar.start_tag()
          for ch in self.top_grammar.children:
              res += ch.serialize()
          res += self.tree.serialize()
          for d in self.global_defs:
              res += self.global_defs[d].serialize()
          for i in self.identities:
              res += self.identities[i].serialize()
          return res + self.top_grammar.end_tag()
docs: Return the string representation of the receiver.