code: func (pis PeerIdentitySet) Filter(filter PeerIdentityFilter) PeerIdentitySet {
  	var result PeerIdentitySet
  	for _, id := range pis {
  		if filter(id) {
  			result = append(result, id)
  		}
  	}
  	return result
  }
docs: // Filter filters identities based on predicate, returns new  PeerIdentitySet
  // with filtered ids.