doc-src/TutorialI/Misc/arith4.thy

setsum is now overloaded on plus_ac0

(*<*)
theory arith4 = Main:;
(*>*)
lemma "\\<not> m < n \\<and> m < n+1 \\<Longrightarrow> m = n";
(**)(*<*)
oops;
end
(*>*)