code: GBSNode leftMostChild(
      InsertStack    stack)
    {
      stack.start(this, "GBSNode.leftMostChild");
      GBSNode l = leftChild();
      GBSNode lastl = this;
      while (l != null)
      {
        stack.push(0, l);
        lastl = l;
        l = l.leftChild();
      }
      return lastl;
    }
docs: Find the left-most child of a node by following all of the
  left children down to the bottom of the tree.
  
  @param stack The InsertStack that is used to record the traversal.
  
  @return The left most child at the bottom of the tree.