code: func HasListenerWithAddress(addr string) bool {
  	instancesMu.Lock()
  	defer instancesMu.Unlock()
  	for _, inst := range instances {
  		for _, sln := range inst.servers {
  			if listenerAddrEqual(sln.listener, addr) {
  				return true
  			}
  		}
  	}
  	return false
  }
docs: // HasListenerWithAddress returns whether this package is
  // tracking a server using a listener with the address
  // addr.