@rule('<STR_LIT>')<EOL><INDENT>def visit_optional_slice_step(self, _, children):<DEDENT>
return children[<NUM_LIT:0>] if children else None<EOL>
The optional slice step for a slice (to be in bracket).

        Arguments
        ---------
        _ (node) : parsimonious.nodes.Node.
        children : list
            - 0: an optional number to use as the ``step`` attribute of a ``slice`` object,
                 got from the ``SLICE_STEP`` rule (which expects a colon and/or a number)

        Returns
        -------
        number or None
            The number to use as the ``step`` attribute of a ``slice`` object.

        Example
        -------

        >>> SliceParserMixin('', default_rule='OPTIONAL_SLICE_STEP').data

        >>> SliceParserMixin(':', default_rule='OPTIONAL_SLICE_STEP').data

        >>> SliceParserMixin(':1', default_rule='OPTIONAL_SLICE_STEP').data
        1