IIRC boost::unordered_map/set offered those, and I'm pretty sure the API was taken more or less verbatim from that. I suspect some desire of compatibility was t

IIRC boost::unordered_map/set offered those, and I'm pretty sure the API was taken more or less verbatim from that. I suspect some desire of compatibility was to blame, but don't know. Hopefully in the future mistakes of these sort will be avoided, but that might be asking for too much (certainly std::optional opted to break compat with boost's (`std::optional<T&>` is disallowed) so there's some hope).