code: func PayloadLen(plen int) NetworkChecker {
  	return func(t *testing.T, h []header.Network) {
  		t.Helper()
  
  		if l := len(h[0].Payload()); l != plen {
  			t.Errorf("Bad payload length, got %v, want %v", l, plen)
  		}
  	}
  }
docs: // PayloadLen creates a checker that checks the payload length.