code: public Iterator<BplusTree.TreeEntry<K, V>> iterator() {
  		return new TreeIterator<BplusTree.TreeEntry<K, V>>(this);
  	}
docs: Note that the iterator cannot be guaranteed to be thread-safe as it is,
  generally speaking, impossible to make any hard guarantees in the
  presence of unsynchronized concurrent modification.
  
  @returns iterator over values in map