Term-rewriting
In mathematics, computer science, and logic, rewriting covers a wide range of (potentially non-deterministic) methods of replacing subterms of a formula with other terms. What is considered are rewriting systems (also known as rewrite systems, rewrite engines or reduction systems). In their most basic form, they consist of a set of objects, plus relations on how to transform those objects.
Rewriting can be non-deterministic. One rule to rewrite a term could be applied in many different ways to that term, or more than one rule could be applicable. Rewriting systems then do not provide an algorithm for changing one term to another, but a set of possible rule applications. When combined with an appropriate algorithm, however, rewrite systems can be viewed as computer programs, and several declarative programming languages are based on term rewriting.
In logic, the procedure for obtaining the conjunctive normal form (CNF) of a formula can be conveniently written as a rewriting system. The rules of such a system would be:
where the symbol () indicates that an expression matching the left hand side of the rule can be rewritten to one formed by the right hand side. In this system, we can perform a rewrite from left to right only when the logical interpretation of the left hand side is equivalent to that of the right.
...
Wikipedia