formation rules

The formation rules of a logical calculus or formal language lay down which sequences of expressions are to count as well formed. Typically these rules are recursive in character, and contain three kinds of clauses. There will be rules defining a class of basic or atomic well-formed formulae; then rules allowing that well-formed formulae may be made out of others by specific operations. Finally there is a closure condition, or rule saying that anything not generated by the first two kinds of rule is disallowed.