@rule('<STR_LIT>')<EOL><INDENT>def visit_optional_nb(self, _, children):<DEDENT>
return children[<NUM_LIT:0>] if children else None<EOL>
Return a int of float (see the NB rule) or None.

        Arguments
        ---------
        _ (node) : parsimonious.nodes.Node.
        children : list, unused

        Result
        ------
        int or float or None

        Example
        -------

        >>> BaseParser('', default_rule='OPTIONAL_NB').data

        >>> BaseParser('1', default_rule='OPTIONAL_NB').data
        1