docs: def p_expr_exit(p):
      '''expr : EXIT
              | EXIT LPAREN RPAREN
              | EXIT LPAREN expr RPAREN'''
      if len(p) == 5:
          p[0] = ast.Exit(p[3], lineno=p.lineno(1))
      else:
          p[0] = ast.Exit(None, lineno=p.lineno(1))