I don't have a use case off the top of my head, I just have some vague memory that it works this way :-).

Honestly the more I think about it the more I suspect that the actual clean solution is to segregate the spanning-slice behaviour into an OrderedIndex type/type hierarchy (which would also enforce that index elements are sorted, might at some point add the option of to skipping the hash table entirely in favor of binary search's decent performance with vastly smaller memory usage, etc.). This thing where we try to guess which semantics are wanted by inspecting the values is pretty uncomfortable.