code: def add_route(self, target_id, stream):
          
          LOG.debug('%r.add_route(%r, %r)', self, target_id, stream)
          assert isinstance(target_id, int)
          assert isinstance(stream, Stream)
  
          self._write_lock.acquire()
          try:
              self._stream_by_id[target_id] = stream
          finally:
              self._write_lock.release()
docs: Arrange for messages whose `dst_id` is `target_id` to be forwarded on
          the directly connected stream for `via_id`. This method is called
          automatically in response to :data:`mitogen.core.ADD_ROUTE` messages,
          but remains public while the design has not yet settled, and situations
          may arise where routing is not fully automatic.