T5.6 8 of 9
Finally, we should look at a problem involving ~E. This rule may be used when an accessible line has main connective tilde. But it is often a good idea to use other rules first. However, if no other means presents itself if you're getting desperate! just use assume the negation of your current goal sentence and derive any contradiction.
Often problems leading to desperation have premises which are negations of molecular sentences. For instance, how would we show the following valid?
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From the premise '~(~AvB)' deduce 'A'.