code: func (s *Session) RemoveParty(pid ID) bool {
  	for i := range s.Parties {
  		if s.Parties[i].ID == pid {
  			s.Parties = append(s.Parties[:i], s.Parties[i+1:]...)
  			return true
  		}
  	}
  	return false
  }
docs: // RemoveParty helper allows to remove a party by it's ID from the
  // session's list. Returns 'false' if pid couldn't be found