docs: public <T extends Widget & Checkable> List<Integer> getCheckedWidgetIndexes() {
          List<Integer> checked = new ArrayList<>();
          List<Widget> children = getChildren();
  
          final int size = children.size();
          for (int i = 0, j = -1; i < size; ++i) {
              Widget c = children.get(i);
              if (c instanceof Checkable) {
                  ++j;
                  if (((Checkable) c).isChecked()) {
                      checked.add(j);
                  }
              }
          }
  
          return checked;
      }