Jerry Rubin's "Yippie Manifesto" is hardly a watershed document. Most "of the young whom it celebrated seven years ago would now find its callow enthusiasms embarrassing and its view of the relations among the time categories in which we live worthy only of those seven-year-olds who, the Manifesto confidently predicted, would settle "the war between THEM and US." Nevertheless, it is still useful as a display piece, in which one can see in exaggerated definition the enduring popular conviction that the past, being "back there," is separable from the present and that therefore Utopia ("HEAVEN NOW") is a real possibility.