Well-formed formulae

The set of all well-formed formulae is inductively defined as: ?

• All propositional symbols are atomic formulae.
• If $A$ is a formula, so is $\neg A$.
• If $A$ and $B$ are formulae, then do is any combination of: $(A\wedge B),(A\vee B),(A\to B),(A⟺B)$
• Nothing else is a formula.