@rule('<STR_LIT>')<EOL><INDENT>def visit_null(self, *_):<DEDENT>
return None<EOL>
Return ``None`` for the string "null", "none", or "nil" (case insensitive)

        Arguments
        ---------
        _[0] (node) : parsimonious.nodes.Node.
        _[1] (children) : list, unused

        Returns
        -------
        None

        Example
        -------

        >>> BaseParser('null', default_rule='NULL').data is None
        True
        >>> BaseParser('Null', default_rule='NULL').data is None
        True
        >>> BaseParser('NULL', default_rule='NULL').data is None
        True
        >>> BaseParser('nil', default_rule='NULL').data is None
        True
        >>> BaseParser('Nil', default_rule='NULL').data is None
        True
        >>> BaseParser('NIL', default_rule='NULL').data is None
        True
        >>> BaseParser('none', default_rule='NULL').data is None
        True
        >>> BaseParser('None', default_rule='NULL').data is None
        True
        >>> BaseParser('NONE', default_rule='NULL').data is None
        True