var last_feature_id="feature1";
                function displayFeature(myid)
                {
                  var element;
                  var last_element;
                  var this_id="feature"+myid;

                  element=document.getElementById(this_id);
                  if (element != null)
                  {
                    last_element=document.getElementById(last_feature_id);
                    if (last_element != null)
                    {
                      last_element.style.display="none";
                    }
                    else
                      alert("last_element is NULL");
                    element.style.display="block";
                    last_feature_id="feature"+myid;
                  }
                  else
                    alert("no element : feature"+myid)
                }