Implementation [of tag systems]
With the rules for case (a) on page 94 given for example by
{2, {{0, 0} {1, 1}, {1, 0} {}, {0, 1} {1, 0}, {1, 1} {0, 0, 0}}}
the evolution of a tag system can be obtained from
TSEvolveList[{n_, rule_}, init_, t_] := NestList[If[Length[#] < n, {}, Join[Drop[#, n], Take[#, n] /. rule]]&, init, t]
An alternative implementation is based on applying to the list at each step rules such as
{{0, 0, s___} {s, 1, 1}, {1, 0, s___} {s}, {0, 1, s___} {s, 1, 0}, {1, 1, s___} {s, 0, 0, 0}}
There are a total of ((kr + 1 - 1)/(k - 1))kn possible rules if blocks up to length r can be added at each step and k colors are allowed. For r = 3, k = 2 and n = 2 this is 50,625.