function superprefetcher_xmlhttpRequester(unsafeContentWin, chromeWindow) {
	this.unsafeContentWin = unsafeContentWin;
	this.chromeWindow = chromeWindow;
}

// this function gets called by user scripts in content security scope to
// start a cross-domain xmlhttp request.
//
// details should look like:
// {method,url,onload,onerror,onreadystatechange,headers,data}
// headers should be in the form {name:value,name:value,etc}
// can't support mimetype because i think it's only used for forcing
// text/xml and we can't support that
superprefetcher_xmlhttpRequester.prototype.contentStartRequest = function(details) {
	// important to store this locally so that content cannot trick us up with
	// a fancy getter that checks the number of times it has been accessed,
	// returning a dangerous URL the time that we actually use it.
	var url = details.url;

	// make sure that we have an actual string so that we can't be fooled with
	// tricky toString() implementations.
	if (typeof url != "string") {
		throw new Error("Invalid url: url must be of type string");
	}

	var ioService=Components.classes["@mozilla.org/network/io-service;1"]
		.getService(Components.interfaces.nsIIOService);
	var scheme = ioService.extractScheme(url);

	// This is important - without it, GM_xmlhttpRequest can be used to get
	// access to things like files and chrome. Careful.
	switch (scheme) {
		case "http":
		case "https":
		case "ftp":
			this.chromeWindow.setTimeout(
				superprefetcher_gmCompiler.hitch(this, "chromeStartRequest", url, details), 0);
			break;
		default:
			throw new Error("Invalid url: " + url);
	}
}

// this function is intended to be called in chrome's security context, so
// that it can access other domains without security warning
superprefetcher_xmlhttpRequester.prototype.chromeStartRequest=function(safeUrl, details) {
	var req = new this.chromeWindow.XMLHttpRequest();

	this.setupRequestEvent(this.unsafeContentWin, req, "onload", details);
	this.setupRequestEvent(this.unsafeContentWin, req, "onerror", details);
	this.setupRequestEvent(this.unsafeContentWin, req, "onreadystatechange", details);

	req.open(details.method, safeUrl);

	if (details.headers) {
		for (var prop in details.headers) {
			req.setRequestHeader(prop, details.headers[prop]);
		}
	}

	req.send(details.data);
}

// arranges for the specified 'event' on xmlhttprequest 'req' to call the
// method by the same name which is a property of 'details' in the content
// window's security context.
superprefetcher_xmlhttpRequester.prototype.setupRequestEvent =
function(unsafeContentWin, req, event, details) {
	if (details[event]) {
		req[event] = function() {
			var responseState = {
				// can't support responseXML because security won't
				// let the browser call properties on it
				responseText:req.responseText,
				readyState:req.readyState,
				responseHeaders:(req.readyState==4?req.getAllResponseHeaders():''),
				status:(req.readyState==4?req.status:0),
				statusText:(req.readyState==4?req.statusText:'')
			}

			// Pop back onto browser thread and call event handler.
			// Have to use nested function here instead of GM_hitch because
			// otherwise details[event].apply can point to window.setTimeout, which
			// can be abused to get increased priveledges.
			new XPCNativeWrapper(unsafeContentWin, "setTimeout()")
				.setTimeout(function(){details[event](responseState);}, 0);
		}
	}
}