@Override
  public void loadReferrers(final String threadId,
                            final String frameId,
                            final PyReferringObjectsValue var,
                            final PyDebugCallback<? super XValueChildrenList> callback) {
    GetReferrersCommand cmd = new GetReferrersCommand(this, threadId, frameId, var);

    cmd.execute(new PyDebugCallback<>() {
      @Override
      public void ok(List<PyDebugValue> value) {
        XValueChildrenList list = new XValueChildrenList();
        for (PyDebugValue v : value) {
          list.add(v);
        }
        callback.ok(list);
      }

      @Override
      public void error(PyDebuggerException exception) {
        callback.error(exception);
      }
    });
  }
@Override
  public void loadReferrers(final String threadId,
                            final String frameId,
                            final PyReferringObjectsValue var,
                            final PyDebugCallback<? super XValueChildrenList> callback)