Disjunction of Conjunctions

Theorem

 * $\left({p \land q}\right) \lor \left({r \land s}\right) \vdash p \lor r$

Proof

 * align="right" | 8 ||
 * align="right" | 1
 * $p \lor r$
 * $\lor \mathcal E$
 * 1, 2-4, 5-7
 * 1, 2-4, 5-7