code: function (possibleVal, isChecked, index) {
              var $listItem;
              var $label;
              var id = this.inputId;
              var attributes = { type: 'checkbox', value: possibleVal.value};
  
              if (this.addId) { extend(attributes, { name: id, id: (id + '-' + index) }); }
              attributes = extend(attributes, this.inputAttrs);
              if (this.inputClass) { attributes.class = this.inputClass; }
              if (isChecked) { attributes.checked = 'checked'; }
              $listItem = Backbone.$('<input>').attr(attributes);
              $label = Backbone.$('<label>').attr(defaults(this.labelAttrs || {}, { 'class': 'checkbox' }));
              return $label.append($listItem).append(possibleVal.display);
          }
docs: Renders and returns a single checkbox in a CheckList
  @memberOf Backbone.fields.CheckListView#
  @param {string} possibleVal the key from possibleVals
  @param {boolean} isChecked if the box should be checked or not
  @param {number} index the index of the checkbox
  @return {$}