Five cases for the obit of a collision fragment. (a) The particle is bound. (b), (c) The particle collides with the central mass. (d), (e) The particle leaves the system. For long intervals between collisions (T_coll ≫ T), cases ((b)≫(e)) can be dealt with by removing the particle. Otherwise, the fragments need to be stored and paired in a similar manner to the bound particles.