code: private GBSNode leftMostChild(
                                   DeleteStack    stack)
      {
          GBSNode       p;
          p = _index.root();             /* Root of tree                          */
          GBSNode lastl = null;          /* Will point to left-most child         */
          if (p != null)                 /* Root is not null, we have a tree      */
          {
              /* Remember father of root               */
              stack.start(_index.dummyTopNode(), "GBSIterator.leftMostChild");
              lastl = leftMostChild(stack, p);
          }
  
          return lastl;
      }
docs: Find the left-most child of the tree by following all of the
  left children down to the bottom of the tree.
  
  @param stack The DeleteStack that is used to record the traversal.
  
  @return The left most child at the bottom of the tree or null
  if the tree is empty.
  
  @exception OptimisticDepthException if the depth of the traversal
  exceeds GBSTree.maxDepth.