docs: def _send_frame(self, connection, frame):
          """
          Sends a frame to a specific subscriber connection.
  
          (This method assumes it is being called from within a lock-guarded public
          method.)
  
          @param connection: The subscriber connection object to send to.
          @type connection: L{coilmq.server.StompConnection}
  
          @param frame: The frame to send.
          @type frame: L{stompclient.frame.Frame}
          """
          assert connection is not None
          assert frame is not None
  
          self.log.debug("Delivering frame %s to connection %s" %
                         (frame, connection))
  
          if connection.reliable_subscriber:
              if connection in self._pending:
                  raise RuntimeError("Connection already has a pending frame.")
              self.log.debug(
                  "Tracking frame %s as pending for connection %s" % (frame, connection))
              self._pending[connection] = frame
  
          connection.send_frame(frame)