The city's a wonderful if filthy and occasionally terrifying ""total learning experience"" for Umberto, a Mexican youngster, in this lurching farce/comedy with poster homiletics and a setting of New York City's subway tunnels. The characters are as ritualized as the more arresting subway graffiti: a spinster librarian/scholar who imports Umberto to absorb the city's past and present with his virgin eye; Umberto's tutor, Snake (Dr. I.R.T.), a black Yale drop-out and draft. dodger, who writes songs; an unloved street youth who drops acid; etc. And among the pipes and abandoned tracks, assorted oddballs and survivors. Umberto rides the rails, witnesses a murder plot, is hunted by a junkie, ""the Claw,"" and finally, in the clear, helps the librarian dig up an ancient Indian canoe. It all ends sunny side up; and in spite of the rhetorical overkill (the librarian's academese and Umberto's crocus English), it's a benign tour of New York's transport of the lost.