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