%%% Petr Ol\v{s}\'ak: Testing whether two characters form a ligature

\newif\ifligature
\def\testligature #1#2{\setbox0=\hbox{%
   \thickmuskip=1000mu \textfont0=\the\font
   $\mathchar`#1 \mathrel\mathchar`#2$}%
   \ifdim\wd0>500pt \ligaturefalse \else \ligaturetrue \fi}