Sun, 22 Oct 2017 13:12:38 +0200 wenzelm tuned: build hg_graph only once;
Sun, 22 Oct 2017 11:59:44 +0200 nipkow tuned
Sun, 22 Oct 2017 09:10:10 +0200 nipkow derived axiom iffI as a lemma (thanks to Alexander Maletzky)
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