A rule which describes how to transform one symbol, or a set of symbols, or a symbol in context, into other symbols. In theory any formal language can be expressed in transformation rules. For example, let's assume that all upper-case words are terminals and everything else isn't. Then, consider the following set of rules:

s ::= verb ME adveb
verb ::= FUCK | SCREW | SMACK
This could produce the sentences FUCK ME HARDER or SMACK ME WITH A CHAINSAW, among others.

A common way to express these rules is using Backus Normal Form.

When referring to transformation rules, the left side means the side that describes what is transformed, and the right side means the side that describes what it can be transformed into.

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