private void prepare() {
    final DiffElement<?> src = getSource();
    final DiffElement<?> trg = getTarget();
    if (src instanceof ComparableDiffElement) {
      ((ComparableDiffElement)src).prepare(trg);
    }
    if (trg instanceof ComparableDiffElement) {
      ((ComparableDiffElement)trg).prepare(src);
    }
    for (DTree tree : getChildren()) {
      tree.prepare();
    }
  }
private void prepare()