I'm a wimp

Coming home on the tube / subway today, I sat down next to two guys who, it quickly became clear, were speaking Hebrew.
I couldn’t hear them properly because of the sound of the train (not that I was trying to... ahem), but at some point, I caught the end of one of their exchanges.
The guy sitting next to me (in Hebrew): "Who?"
The other guy: "The girl sitting next to you."
(Ie. -- Me.)
The guy sitting next to me: "Imagine we were talking about someone [in the feminine -- MS] on the train and it turned out she was Israeli?"
This, of-course, is the point at which I should have come out with some cutting remark about how he doesn’t have to imagine too hard – it’s happening to him right now. Just like the old urban myth about the two guys on the NY subway who call some woman a cow in Hebrew, and before she gets off, she turns around and moos at them. Unfortunately, I was so busy trying to work out whether what they’d said about me had been positive or negative that I completely missed the moment. They got off the train before I managed to recuperate.
Now I'm kicking myself for not saying anything. Any suggestions for how I should have handled it? I want to be prepared for next time (and there probably will be a next time, seeing how full this city is of Israelis).