A sequential substitution system is a substitution system in which a string is scanned from left to right for the first occurrence of the first rule pattern. If the pattern is found, the rule is applied and processing advances to the next step. If the first rule pattern does not occur, the string is scanned from left to right for the first occurrence of the second rule pattern. If the pattern is found, the rule is applied and processing advances to the next step, and so on. If none of the rule patterns match at some step, the string repeats indefinitely for all subsequent steps. For example, consider the single rule A->B and the initial string A B A, illustrated above. The first step yields B B A, the second step yields B B B, and from there on the system repeats since there are no more matches of the pattern rule.

multiway system | substitution system