code: def feed(self, data):
          
          assert not self.closed
          for mark, special in self.escape_map:
              assert mark not in data
              data = data.replace(special, mark)
          super().feed(data)
docs: We need to prevent insertion of some special HTML characters
          (entity refs and maybe more).  It's hard to prevent the state machine
          from blowing up with them in the data, so we temporarily replacing
          them with a reserved unicode value based on the PAU private space.