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function loadScript(url, onLoaded) {
    var loaded = false;
    var scriptTag = document.createElement('script');
    scriptTag.src = url;
    scriptTag.onload = scriptTag.onreadystatechange = function(){
        if(!loaded && onLoaded) {
            loaded = true;
            onLoaded();
            scriptTag.onload = scriptTag.onreadystatechange = null;
        }
    }
    document.head.appendChild(scriptTag);
}