gistfile1.txt

var MS = function(config) {

    // Since you won't be mixing in Y.ModelSync.REST 
    // as a separate extensions

    Y.ModelSync.REST.apply(this, arguments);
};

Y.mix(MS, Y.ModelSync.REST); // Mainly to pull over ModelSync.REST.ATTRS, but maybe other stuff also.
Y.mix(MS.prototype, Y.ModelSync.REST.prototype);

MS.prototype.getURL : function (action, options) {

    if (Lang.isArray(this._shelves)) {
        return this.root;
    }

    // other conditions here
    return Y.ModelSync.REST.prototype.getURL.apply(this, arguments);
};

MS.prototype.sync = function() {

    if (someoverridecondition) {
        // new logic
    } else {
        Y.ModelSync.REST.prototype.sync.apply(this, arguments);
    }
};

Y.GalleryModelSync = MS;