gistfile1.js

var greasemonkey = (typeof unsafeWindow != 'undefined');

if (greasemonkey)
	Foo.setup(); // use @resource
else {
	// referred to jQuery
	var head = document.getElementsByTagName('head')[0];
	var script = document.createElement('script');
	script.src = 'http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js';

	var done = false;
	script.onload = function() {
		if (done) return;
		done = true;

		Foo.setup();
	};
	head.insertBefore(script, head.firstChild);
}