code: func MembershipByCert(consenters map[uint64]*etcdraft.Consenter) map[string]uint64 {
  	set := map[string]uint64{}
  	for nodeID, c := range consenters {
  		set[string(c.ClientTlsCert)] = nodeID
  	}
  	return set
  }
docs: // MembershipByCert convert consenters map into set encapsulated by map
  // where key is client TLS certificate