Tue, 16 Jul 2013 10:18:25 +0200 nipkow added exercise
Mon, 15 Jul 2013 15:50:39 +0200 traytel killed unused theorems
Mon, 15 Jul 2013 14:23:51 +0200 traytel eliminate duplicated theorems (thanks to "Auto solve_direct" in jEdit)
Mon, 15 Jul 2013 11:29:19 +0200 wenzelm tuned specifications and proofs;
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