Nearby, a graceful white swan no kidding perches on a perfect little waterfall. But here on the patio is Walter Matthau, whose magnificently rumpled presence gives even this place a certain air du pool hall. I can no longer have a bowel movement without a lot of help. Such unfettered discourse has made Matthau, throughout his 46 years in show business and 56 filmsconsistently good copy. But his career has been less consistent. Then last summer he returned to the big screen, as Mr. Wilson in Dennis the Menace. Grumpy makes his comeback official. Plus it has the best end credits running, with Burgess Meredith, age 84, rattling off filthy euphemisms for sexual intercourse-lines that both Lemmon and Matthau refused to say. It has every country and every emotion and every person that ever lived in it. Sorrow also flows through those astounding crevices.