  We present a simple analytical model for studying the collapse of an
ultracompact stellar object (regular black hole mimicker with infinite redshift
surface) to form a (integrable) black hole, in the framework of General
Relativity. Both initial and final configurations have the same ADM mass, so
that the transition represents an internal redistribution of matter without
emission of energy. The model, despite being quite idealized, can be viewed as
a good starting point to investigate near-horizon quantum physics
