The documentation of the OPmac is: opmac-u.tex,pdf ... user documentation (in Czech) opmac-u-en.tex,pdf ... user documentation (in English) opmac-d.tex ... technical documentation (in Czech) See http://petr.olsak.net/opmac.html for more information. This is part of csplain package. The version of the OPmac can be found in the file tex/csplain/opmac/opmac.tex at line 8.