code: func (n *Net) Listen(nett, laddr string) (net.Listener, error) {
  	switch nett {
  	default:
  		return nil, net.UnknownNetworkError(nett)
  	case "tcp", "tcp4", "tcp6":
  		addr, err := net.ResolveTCPAddr(nett, laddr)
  		if err != nil {
  			return nil, err
  		}
  		return n.ListenTCP(nett, addr)
  	case "unix", "unixpacket", "invalid_unix_net_for_test":
  		addr, err := net.ResolveUnixAddr(nett, laddr)
  		if err != nil {
  			return nil, err
  		}
  		return n.ListenUnix(nett, addr)
  	}
  }
docs: // Listen announces on the local network address laddr. The network net must be
  // a stream-oriented network: "tcp", "tcp4", "tcp6", "unix" or "unixpacket". It
  // returns an inherited net.Listener for the matching network and address, or
  // creates a new one using net.Listen.