Examples of tag systems in which at each step two elements are removed from the beginning of the sequence and then, based on what these elements are, a specified block of new elements is added to the end of the sequence. (The three dots in the representation of each rule stand for the rest of the elements in the sequence.) The pictures at the top show the first hundred steps in evolution according to various rules starting from a pair of black elements. The plots show the total lengths of the sequences obtained in each case. Note that in case (c), all the elements are eventually removed from the sequence.

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