WFFs of PropLog of Length 1
- The letters of the formal grammar of propositional logic $\mathcal L_0$
- The tautology symbol $\top$
- The contradiction symbol $\bot$.
We refer to the rules of formation.
From $\mathbf W: TF$, $\top$ and $\bot$ (both of length 1) are WFFs.
From $\mathbf W: \mathcal P_0$, all elements of $\mathcal P_0$ (all of length 1) are WFFs.
Every other rule of formation of the formal grammar of propositional logic consists of an existing WFF in addition to at least one other primitive symbol.
Hence the result.