Rule of Idempotence/Disjunction/Formulation 1/Reverse Implication

Theorem

 * $p \lor p \vdash p$

Proof

 * align="right" | 3 ||
 * align="right" | 1
 * $p$
 * $\lor \mathcal E$
 * 1, 2-2, 2-2
 * }
 * }
 * }