code: private void stackStart(
      GBSNode     node)
    {
      _idx = 0;
      _cidx = 0;
      _state[_cidx] = 0;
      _node[_cidx]  = node;
      _bpidx = _cidx + 1;
      _maxIdx = _cidx;
    }
docs: Start the stack with the root of the tree.