code: func (g *gossipServiceImpl) PeerFilter(channel common.ChainID, messagePredicate api.SubChannelSelectionCriteria) (filter.RoutingFilter, error) {
  	gc := g.chanState.getGossipChannelByChainID(channel)
  	if gc == nil {
  		return nil, errors.Errorf("Channel %s doesn't exist", string(channel))
  	}
  	return gc.PeerFilter(messagePredicate), nil
  }
docs: // PeerFilter receives a SubChannelSelectionCriteria and returns a RoutingFilter that selects
  // only peer identities that match the given criteria, and that they published their channel participation