Z:gnu-www-ja-gpl-2.0-faq--8f6680-As it happens, Bison can also/en

As it happens, Bison can also be used to develop non-free programs. This is because we decided to explicitly permit the use of the Bison standard parser program in Bison output files without restriction. We made the decision because there were other tools comparable to Bison which already permitted use for non-free programs.