Substitution systems [from cellular automata]

Given a substitution system with rules in the form such as {1  {0}, 0  {0, 1}} used on page 889, the rules for a cellular automaton which emulates it are obtained from

SSToCA[rules_] := {{b, b, p[x_, _]}  s[x], {_, s[v : (0 | 1)], p[x_, _]}  p[v, x], {_, p[_, y_], _}  s[y], {_, s[v : (0 | 1)], _m}  m[v], {s[0 | 1], m[v : (0 | 1)], _}  s[v], {b, m[v : (0 | 1)], _}  r[v], {_, r[v : (0 | 1)], _}  (Replace[v, rules] /. {{x_}  s[x], {x_, y_}  p[x, y]}), {_r, s[v : (0 | 1)], _}  r[v], {_r, b, _}  m[b], {s[0 | 1], m[b], _}  b, {_, v_, _}  v}

where specific values for cells can be obtained from

{b  0, s[0]  1, m[0]  2, p[0, 0]  3, r[0]  4, p[0, 1]  5, p[1, 0]  6, r[1]  7, p[1, 1]  8, m[1]  9, m[b]  10, s[1]  11}

An initial condition consisting of a single element with color i in the substitution system is represented by m[i] surrounded by b's in the cellular automaton. The specific definition given above works for neighbor-independent substitution systems whose elements have two possible colors, and in which each element is replaced at each step by at most two new elements.