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And having done this, the next stage is to get such a tag system to emulate a Turing machine. The pictures on the next page illustrate how this can be done. But at least with the particular construction shown, the resulting Turing machine can only have cells with two possible colors. The pictures below demonstrate, however, that such a Turing

Captions on this page:

Emulating a Turing machine with a tag system that depends only on the first element at each step. The configuration of cells on each side of the head in the Turing machine is treated as a base 2 number. At the steps indicated by arrows the tag system yields sequences of dark cells with lengths that correspond to each of these numbers.

Turing machines with two colors emulating ones with more colors.

From Stephen Wolfram: A New Kind of Science [citation]