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