code: func AckNum(seq uint32) TransportChecker {
  	return func(t *testing.T, h header.Transport) {
  		t.Helper()
  		tcp, ok := h.(header.TCP)
  		if !ok {
  			return
  		}
  
  		if s := tcp.AckNumber(); s != seq {
  			t.Errorf("Bad ack number, got %v, want %v", s, seq)
  		}
  	}
  }
docs: // AckNum creates a checker that checks the ack number.