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