function width2screensFor(classes) {
        var width = null;
        var width2screens = {};
        while (true) {
            var match = COL_REGEX_G.exec(classes);
            if (!match) {
                break;
            }
            var screen = match[1];
            width = match[2];
            var screens = width2screens[width];
            if (!screens) {
                screens = width2screens[width] = [];
            }
            screens.push(SCREEN2NUM[screen]);
        }

        for (width in width2screens) {
            if (Object.prototype.hasOwnProperty.call(width2screens, 'width')) {
                width2screens[width].sort(compareNums);
            }
        }

        return width2screens;
    }
@param {string} classes The "class" attribute of a DOM node
@returns {Object.<string, integer[]>} Object mapping grid column widths (1 thru 12) to sorted arrays of screen size numbers (see SCREEN2NUM)
     Widths not used in the classes will not have an entry in the object.