// n. The most usual formalism for expressing a structure-building process in a formal grammar. In its most general form, a rewrite rule is simply an instruction to rewrite some string of categories (conventionally given on the left of an arrow) as some other string of categories (on the right); the only general restriction is that the left side may not be null.
Since, in the most general case, any arbitrary string can be rewritten as any other arbitrary string, the derivation of a terminal string from an initial symbol cannot, in general, be represented as a tree, but only as a sequence of strings, and hence the rewrite rule formalism is most appropriate to discussions of weak generative capacity: consider, for example, the impossibility of interpreting the rule A B d Ed A C c E d as an unambiguous tree. When, however, sufficiently severe restrictions are placed upon the permitted rules, the sequence of strings may be replaced by the more informative constituent structure trees. This is true of the subset of rewrite rules called context-free rules, and, providing certain precautions are taken, it is also true of the less restricted subset called context-sensitive rules.
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/ n. The most usual formalism for expressing a structure-building process in a formal grammar. In its most general form, a rewrite rule is simply an instruction to rewrite some string of categories (conventionally given on the left of an arrow) as some other string of categories (on the right); the only general restriction is that the left side may not be null. 
d A C c E d as an unambiguous tree. When, however, sufficiently severe restrictions are placed upon the permitted rules, the sequence of strings may be replaced by the more informative constituent structure trees. This is true of the subset of rewrite rules called 
