code: def p_system_sentence(self, t):
          
  
          if len(t) == 3:
              t[0] = system(t[2], reference=True, line=t.lineno(1))
          else:
              t[0] = system(t[2], t[4], line=t.lineno(1))
docs: system_sentence : SYSTEM VAR
                             | SYSTEM VAR LPAREN features RPAREN