Hi! I'm just reading Wilfrid Hodges's book Logic, chapter 24. Properties of Semantic Entailment. I'm a bit puzzled by the following paragraphs concerning the Extension Theorem:(adsbygoogle = window.adsbygoogle || []).push({});

http://i83.photobucket.com/albums/j315/dobry_den/extension_theorem.jpg

What if X contains formulae A, A->B and Y contains just B' (negation of B). Then the resulting set X,Y is semantically inconsistent and therefore the theorem isn't true.

I'm probably wrong, but I can't see where...

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# Extension Theorem

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