code: public static <DATATYPE extends Comparable <? super DATATYPE>, ITEMTYPE extends ITreeItem <DATATYPE, ITEMTYPE>> void sort (@Nonnull final IBasicTree <DATATYPE, ITEMTYPE> aTree)
    {
      _sort (aTree, (o1, o2) -> o1.getData ().compareTo (o2.getData ()));
    }
docs: Sort each level of the passed tree with the specified comparator. This
  method assumes that the values in the tree item implement the
  {@link Comparable} interface.
  
  @param aTree
  The tree to be sorted.
  @param <DATATYPE>
  The tree item data type
  @param <ITEMTYPE>
  The tree item type