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