Examples of substitution systems that have three and four possible colors for each element. The particular rules shown are ones that lead to slow growth in the total number of elements. Note that on each line in each picture, only the order of elements is ever significant: as the insets show, a particular element may change its position as a result of the addition or subtraction of elements to its left. Note that the pattern in case (a) does eventually repeat, while the one in case (b) eventually shows a nested structure.

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