/**
 * Ajax.Request.abort
 * extend the prototype.js Ajax.Request object so that it supports an abort method
 */
Ajax.Request.prototype.abort = function() {
    // if already completed, ignore abort request
    if(this._complete) return; 
    // prevent and state change callbacks from being issued
    this.transport.onreadystatechange = Prototype.emptyFunction;
    // abort the XHR
    this.transport.abort();
    // update the request counter
    if(Ajax.activeRequestCount < 0) Ajax.activeRequestCount--;
};