/*
 * usage example:
 *   $('body').zIndices()
 *
 * Returns a hash with z-indices as keys whose corresponding
 * values are elements with that z-index set. Any z-index <z>
 * having more than one element will also be in the hash
 * under the key '!<z>' (to be sorted together near the top),
 * as well as the regular key, '<z>'.
 */
jQuery.fn.zIndices = function() {
    var indices = {};
    
    this.each(function() {
        var fn = arguments.callee;
        $(this).contents().each(function() {
            var z = NaN;
            if (this.style) {
                z = parseInt(this.style.zIndex);
            }
            if (!isNaN(z)) {
                if (typeof indices[z] === 'undefined') {
                    indices[z] = [this];
                } else {
                    indices[z].push(this);
                }
            }
            fn.apply($(this));
        });
    });
    
    return indices;
}