well-formed formula

A well-formed formula in propositional logic is a formula constructed from the set of rules below:

• An atomic proposition $p$ is well-formed.
• If $p$ is well-formed, then so is $\neg p$
• If $p$ and $q$ are well-formed, then so are $p\wedge q$, $p\vee q$, $p⟹q$, and $p⟺q$.
• If $p$ is well-formed, then so is $(p)$.