Those who decry the lack of avant-garde on local stages will find the gap briefly filled, or, conceivably, deepened and widened, by Belgium's Le Plan K, and its " Le Nu Traverse" at the Washington Project for the Arts.

This group, with two of its five members performing, was formed five years ago in Brussells by Frederick Flamand and Baba (the two visible performers) and Arthur Spilliaert, and it appears to do a great deal of traveling. Since appearing in last summer's New Theater Festival in Baltimore, where I first saw "Le Nu Traverse," the group has been resident in New York's Off-off Broadway area, where it also has prsented "The Penny Arcade," partially drawm from the writings of William S. Burroughs.

Set to music of idiophones, marble Knuckles and mallets, the piece (to be repeated tonight at 8:30) is spoken in French, drawn from words by authors devoted to schizophrenic expressions. The audience is seated on four sides of what appears to be a brown, handwoven, round rug but which actually is hundreds of pebbles which ultimatley will be scattered around the playing area.

Two actors enter, Bab in a leather apron and Flammand in briefs, knee-pads and a helmet. A steel pole unites them with a wooden and metal mannekin, passing through the head of the mannekin. In time the men use the dummy mechanically, Flamand winding up nude and wrestling with it-or secually assulting it, I wouldn't be sure-on the pebbles. To strains of Bach they exit after 45 minutes, and the audience of 50 applauds and then sits. And sits. And sits. Thaths all there is. There isn't any more.

Having now devoted 90 minutes of my life to watching "Le Nu Traverse," I can only report that marvels never cease, that if Le Plan K vows that it concerns schizophrenics so be it and that P. T. Barnum was absolutely right.