docs: GBSNode lowerPredecessor(
      NodeStack     stack)
    {
      GBSNode  r     = leftChild();
      GBSNode  lastr = r;
      if (r != null)
        stack.push(NodeStack.PROCESS_CURRENT, this);
      while (r != null)              /* Find right-most child of left child   */
      {
        if (r.rightChild() != null)
          stack.push(NodeStack.DONE_VISITS, r);
        lastr = r;
        r = r.rightChild();
      }
      return lastr;
    }