Tab.prototype.beforeChildAdded_ = function (child, insertBefore) {
    var firstChild = this.firstChild;
    if (child instanceof zul.wgt.Caption) {
      //caption is always the first child (if exist)
      if (firstChild && firstChild instanceof zul.wgt.Caption && firstChild != child) {
        zk.error('Only one caption is allowed: ' + this.className);
        return false;
      }
    } else if (!(child instanceof zul.wgt.Label)) {
      zk.error('Only caption is allowed: ' + this.className);
      return false;
    }
    return true;
  };