Prove the relevance lemma, which asserts that the truth value of a propositional assertion in a truth table depends only on the values of the propositional atoms actually appearing in that assertion. In other words, if we make a truth table for $$ using irrelevant atoms, then ${\left[\begin{array}{c}\end{array}\right]}_u={\left[\begin{array}{c}\end{array}\right]}_w$ whenever rows $u$ and $w$ agree on the relevant atoms, those appearing in $.$ And furthermore, the resulting value is the same as $[{]}_v$ where $v=wA$ is the restriction of that truth table to use the set $A$ of the relevant atoms only, those actually appearing in $.$