Alternate models [of fracture]

It is straightforward to set up 3-color cellular automata with the same basic idea as in the main text, but in which there is no need for a special cell to represent the crack. In addition, instead of modelling the displacement of atoms, one can try to model directly the presence or absence of atoms at particular positions. And then one can start from a repetitive array of cells, with a perturbation to represent the beginning of the crack.