code: func (t *BTree) DescendGreaterThan(pivot Item, iterator ItemIterator) {
  	if t.root == nil {
  		return
  	}
  	t.root.iterate(descend, nil, pivot, false, false, iterator)
  }
docs: // DescendGreaterThan calls the iterator for every value in the tree within
  // the range (pivot, last], until iterator returns false.