We prove a two-term quasi-classical trace asymptotic formula for the
functions of multi-dimensional Wiener-Hopf operators with discontinuous
symbols. The discontinuities occur on the surfaces which are assumed to be
piece-wise smooth. Such a two-term formula was conjectured by H. Widom in 1982,
and proved by A. V Sobolev for smooth surfaces in 2009.Comment: 15 page