code: func UDP(checkers ...TransportChecker) NetworkChecker {
  	return func(t *testing.T, h []header.Network) {
  		t.Helper()
  
  		last := h[len(h)-1]
  
  		if p := last.TransportProtocol(); p != header.UDPProtocolNumber {
  			t.Errorf("Bad protocol, got %v, want %v", p, header.UDPProtocolNumber)
  		}
  
  		udp := header.UDP(last.Payload())
  		for _, f := range checkers {
  			f(t, udp)
  		}
  		if t.Failed() {
  			t.FailNow()
  		}
  	}
  }
docs: // UDP creates a checker that checks that the transport protocol is UDP and
  // potentially additional transport header fields.