I cry in movies. I'm a rank sentimentalist. But there are different kinds of weeping. You can be reduced by beauty, or at the simply very good, by the wiles of nostalgia, by acts of courage (however fictional), and so on and so forth. Occasionally, however, the tears come out of a kind of shame, and this is what happened to me at the end of Tom Stoppard's new play, ''Leopoldstadt'. And now I have just finished reading the text, and again I can hardly stifle a sob, as I sit among the quiet and the studious in a British Library reading room.