toString(format = Format.EXACT) {
        let data = this.rows().
            map(row => row.
                map(e => e.toString(format)).
                join(format.itemSeparator)).
            join("}" + format.itemSeparator + "{");
        return "{{" + data + "}}";
    }
Returns a text representation of the receiving matrix.
(It uses curly braces so you can paste it into wolfram alpha.)
@param {=Format} format
@returns {!string}