var loaded = false; 
  var old = "NIX";
  var ende = true;   

  function oncl(a){ 
      if (loaded) { 
          ende = false;
          var objOld = document.getElementById(old);
          var objA = document.getElementById(a);
          if( objA ) {
              objA.style.display = 'block';  
          }
          if ( objOld ) {
              objOld.style.display = 'none';  
          }
          old = a 
      }
      return ende; 
  }