Towards a theory of thin self-gravitating crossing shells
  In the case of crossing thin dust shells the momentum conservation law is found. For two crossing isotropic shells it coincides with the 't Hooft-Dray formula. The system of one isotropic and one time-like shell is considered. In this case we found a very simple formula which relate velocities of dust shell before and after crossing. 
V.A. Berezin, A.L. Smirnov (INR, Moscow)