algorithm above. In step (iv) we have assumed that the list of entry points from $B_{\beta'}$ into $B_{\alpha}$ is not empty and, as already mentioned above, this may not be true at the beginning of the simulation (there might have been no collision with that edge up to that time). In that case we have to put the simulations in some of cells on hold at the beginning until we get proper re-entry points and start building databanks on the edges. Note that, at any given time, these databanks could each contain the last re-entry point only, though in that case we may run out of points again and have to put replicas on hold. Thus it is safer to always keep as many points in the databanks as memory allows. However, the databanks do not have to be enlarged indefinitely – at worst we trade memory for CPUs since, the smaller the databanks, the higher the probability that one replica will have to be put on hold for some time. Also note that the quantities used to evaluate the probability in Eq.  have to be computed on-the-fly, and need information from all the cells. This, again, may lead to 