class Proc;def t;call rescue();end;def +@;p t ? '.':f;end;def -@;p t ? f: '.';end;def f;'!'+to_s;end;end

+->{ 10 * 2 == 20 }
-->{ 10 * 2 == 21 }
+->{ 10 * 2 == 22 } # supposed to fail to show how source location indication works