bison generates, from a series of rules, a program for analyzing the structure of text files. Bison is a replacement for yacc (Yet Another Compiler Compiler). yacc is a wrapper for bison, meant for programs that still call yacc instead of bison. It calls bison with the -y option. liby.a is the Yacc library containing implementations of Yacc-compatible yyerror and main functions. This library is normally not very useful, but POSIX requires it.