  We analyse the dynamics of the collision of two spherical massive shells in a
generally spherically symmetric background, obtaining an expression from the
conservation law that imposes a constraint between the different parameters
involved. We study the light-like limit and make some comparisons of the
predictions of our master equation with the results obtained in the case of
collision of light-like shells, like the short life of white holes or the mass
inflation phenomena. We present some particular cases of the constraint
equation.
