[]-slicing. Yields a "random access range" with the content of the tree, always containing zero and the key after
the maximum element as keys. The key after the maximal key is the universe, if the tree is empty or the maximal
contained key is lesser then empty, otherwise the capacity of the tree.
[]-slicing. Yields a "random access range" with the content of the tree, always containing zero and the key after the maximum element as keys. The key after the maximal key is the universe, if the tree is empty or the maximal contained key is lesser then empty, otherwise the capacity of the tree.