doc-src/TutorialI/Misc/arith3.thy

tidied, proving gcd_greatest_iff and using induct_tac

(*<*)
theory arith3 = Main:;
(*>*)
lemma "n*n = n \\<Longrightarrow> n=0 \\<or> n=1";
(**)(*<*)
oops;
end
(*>*)