@NotNull
  public synchronized Collection<UsageNode> getUsageNodes() {
    List<UsageNode> list = new ArrayList<>();
    for (Node n : myChildren) {
      if (n instanceof UsageNode) {
        list.add((UsageNode)n);
      }
    }
    return list;
  }
@NotNull
  public synchronized Collection<UsageNode> getUsageNodes()