@rule('<STR_LIT>')<EOL><INDENT>def visit_optional_args(self, _, children):<DEDENT>
return children[<NUM_LIT:1>]<EOL>
The optional arguments of a filter.

        Arguments
        ---------
        _ (node) : parsimonious.nodes.Node.
        children : list
            - 0: for ``PAR_O`` (opening parenthesis): ``None``.
            - 1: list of instances of subclasses of ``.resources.Arg``.
            - 2: for ``PAR_C`` (closing parenthesis): ``None``.

        Returns
        -------
        list(.resources.Arg)
            List of  instances of subclasses of ``.resources.NamedArg``.

        Example
        -------

        >>> ArgsParserMixin(r'()', default_rule='OPTIONAL_ARGS').data
        []
        >>> ArgsParserMixin(r'(1)', default_rule='OPTIONAL_ARGS').data
        [1]
        >>> ArgsParserMixin(r'(foo="bar")', default_rule='OPTIONAL_ARGS').data
        [foo="bar"]
        >>> ArgsParserMixin(r'(1, foo="bar")', default_rule='OPTIONAL_ARGS').data
        [1, foo="bar"]