code: function hidden(val) {
    // return whether the first element in the set
    // is hidden
    if(val === undefined) {
      return this.attr(attr);
    // hide on truthy
    }else if(val) {
      this.attr(attr, '1');
    // show on falsey
    }else{
      this.attr(attr, null);
    }
    return this;
  }
docs: Modify the hidden attribute.