gistfile1.js

    function loadScript(url,callback) {

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                 script.setAttribute('type','text/javascript');

                 if(script.readyState) {

                      script.onreadystatechange = function() {

                               if(script.readyState == 'loaded' || script.readyState == 'complete' ) {

                                      script.onreadystatechange = null;

                                      callback(); 
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                      }

                 } else {

                      script.onload = function(){

                             callback();
                      } 
                 }   

                 script.setAttribute('src',url);

                 document.getElementsByTagName('head')[0].appendChild(script); 
    };