@rule('<STR_LIT>')<EOL><INDENT>def visit_unnamed_arg(self, _, children):<DEDENT>
return self.PosArg(<EOL>value=children[<NUM_LIT:0>],<EOL>)<EOL>
Unnamed argument of a filter.

        Arguments
        ---------
        _ (node) : parsimonious.nodes.Node.
        children : list
            - 0: value of the unnamed arg

        Returns
        -------
        list(.resources.PosArg)
            Instances of ``.resources.PosArg``.

        Example
        -------

        >>> UnnamedArgsParserMixin(r'1', default_rule='UNNAMED_ARG').data
        1
        >>> UnnamedArgsParserMixin(r'"foo"', default_rule='UNNAMED_ARG').data
        "foo"
        >>> UnnamedArgsParserMixin(r'null', default_rule='UNNAMED_ARG').data
        None
        >>> UnnamedArgsParserMixin(r'FALSE', default_rule='UNNAMED_ARG').data
        False
        >>> UnnamedArgsParserMixin(r'True', default_rule='UNNAMED_ARG').data
        True