code: function(type, walk) {
      var ctl = this.createCtl(type + 'Group');
      ctl.items = [];
      // Tweak selector to use the generated id
      this.selector.id = ctl.id;
      // Append a class
      if (!Array.isArray(this.selector.classes))
        this.selector.classes = [];
      this.selector.classes.push(ctl.type);
      // Emit root element
      this.out.push('<fieldset');
      this.emitSelector();
      this.out.push('>');
      // Emit each item
      var startIdx = walk.position;
      while(walk.hasCurrent()) {
        walk.scrollToEol().skipWhitespaces();
        if (this.atGroupMarker(type, walk)) {
          this.emitGroupItem(ctl, new SubWalker(walk, startIdx, walk.position));
          startIdx = walk.position;
        }
      }
      // Emit the last one
      this.emitGroupItem(ctl, new SubWalker(walk, startIdx, walk.position));
      this.out.push('</fieldset>');
    }
docs: Emits a control group, e.g.:
  
  ```
  <fieldset id="{ctl.id}" class="checkboxGroup">
  <div id="item-{item.id}" class="item">
  <input id="{item.id}"
  name="{ctl.id}"
  type="checkbox"
  value="{item.id}"/>
  <label for="{item.id}">{item.label}</label>
  </div>
  ...
  </fieldset>
  ```