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
s ::= verb ME adveb
verb ::= FUCK | SCREW | SMACK
adverb ::= HARDER | WITH A CHAINSAW
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.