  Diffraction of a high-frequency large-number whispering gallery mode is
studied, which runs along a concave curve turning to a straight line. At the
point of straitening, the curvature of the boundary suffers a jump. The
parabolic equation method is developed in the problem, and asymptotic formulas
are presented for all waves arising in the vicinity of the non-smoothness point
of the boundary. The ``ray skeleton'' of the wavefield is investigated in
detail.
