code: private void countDescendants() throws TTIOException {
          long descendants = 1;
          final long nodeKey = mRtx.getNode().getDataKey();
          if (((ITreeStructData)mRtx.getNode()).hasFirstChild()) {
              mRtx.moveTo(((ITreeStructData)mRtx.getNode()).getFirstChildKey());
              do {
                  descendants += mDescendants.get(mRtx.getNode());
              } while (((ITreeStructData)mRtx.getNode()).hasRightSibling()
                  && mRtx.moveTo(((ITreeStructData)mRtx.getNode()).getRightSiblingKey()));
          }
          mRtx.moveTo(nodeKey);
          mDescendants.put(mRtx.getNode(), descendants);
      }
docs: Count descendants of node (including self).
  
  @throws TTIOException