@rule('<STR_LIT>')<EOL><INDENT>def visit_slice_step(self, _, children):<DEDENT>
return children[<NUM_LIT:2>] if len(children) > <NUM_LIT:2> else None<EOL>
The optional slice step for a slice (to be in bracket).

        Arguments
        ---------
        _ (node) : parsimonious.nodes.Node.
        children : list
            - 0: for ``COL``, the colon character: ``None``
            - 1: for ``WS``, optional white space: ``None``
            - 2: an optional number to use as the ``step`` attribute of a ``slice`` object

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

        Example
        -------

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

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

        >>> SliceParserMixin(': 2', default_rule='SLICE_STEP').data
        2