docs: function(keypath, value, attributes) {
        var selected;
        attributes = $.extend(this._attributesForKeyPath(keypath), attributes);
        selected = attributes.value;
        attributes.value = getStringContent(this.object, value);
        if (selected == attributes.value) {
          attributes.checked = 'checked';
        }
        return simple_element('input', $.extend({type:'radio'}, attributes));
      }