/********************************
 *            AJAX
*********************************/


  function retrieveURL(url) {	
    var cargando=document.getElementById('cargando');
    cargando.style.visibility='';

    if (window.XMLHttpRequest) { // Non-IE browsers
      req = new XMLHttpRequest();
      req.onreadystatechange = processStateChange;
      try {
        req.open("GET", url, true);
      } catch (e) {
        alert(e);
      }
      req.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
      req.send(null);
    } else if (window.ActiveXObject) { // IE
      req = new ActiveXObject("Microsoft.XMLHTTP");
      if (req) {
        req.onreadystatechange = processStateChange;
        req.open("GET", url, true);
        req.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        req.send();
      }
    }
  }

  function processStateChange() {
    if (req.readyState == 4) { // Complete
      if (req.status == 200) { // OK response
          var cargando=document.getElementById('cargando');
          cargando.style.visibility='hidden';
       document.getElementById("contenido").innerHTML = req.responseText;
      } else {
        alert("Problem: " + req.responseText);
      }
    }
  }