public parse_A() {
   ttype = getToken();
   if (ttype == atype)
   {
      parse_A();
      ttype = getToken();
      if (ttype == btype)
            print "A -> aAb";
      else
            syntax_error();
}
else if  (ttype == btype || type == eof)
{
      ungetToken();
      print "A -> epsilon";
   }
   else
      syntax_error();
}