RIGOR AND ECONOMY OF CUT, a marked absence of artifice, the clash of high-tech and traditional fabrics. Helmut Lang's deceptively simple designs lend even the most basic garment a subversive edge. Yet while his use of high-tech materials has been much discussed, the frisson of his interventions has less to do with the techno-club futurism reflected in recent collections by Jean Colonna, Katherine Hamnett, Martin Sitbon, Rifat Ozbek, and Walter von Beirendonck than with his wholly sartorial solutions. Lang's fashions remain autoreferential; he is concerned first and foremost with the formal