Any person recognize the definition of yy?

Given that history questions are considered on topic, I figured I would certainly ask one that's been badgering me for fairly time and also no person I recognize appears to recognize the solution. Does any person recognize what the historic value is of the prefix yy? A lot of the variables and also features created by lex start with it, such as yylval, and also yylex(). I additionally bear in mind seeing an energy called yyrsa someplace, yet can not locate referrals to it anywhere currently. Making inquiries minds need to know!