code: func (l *State) AddAliasCheck(checkID types.CheckID, srcServiceID string, notifyCh chan<- struct{}) error {
  	l.Lock()
  	defer l.Unlock()
  
  	m, ok := l.checkAliases[srcServiceID]
  	if !ok {
  		m = make(map[types.CheckID]chan<- struct{})
  		l.checkAliases[srcServiceID] = m
  	}
  	m[checkID] = notifyCh
  
  	return nil
  }
docs: // AddAliasCheck creates an alias check. When any check for the srcServiceID is
  // changed, checkID will reflect that using the same semantics as
  // checks.CheckAlias.
  //
  // This is a local optimization so that the Alias check doesn't need to use
  // blocking queries against the remote server for check updates for local
  // services.