> As a general rule, a key should be a simple value.
> Try to avoid using complex structures as keys.

Or rather, if you are going to use objects as keys, think of them as
each *instance* being a unique key, not "this object happens to have
the same contents as that other one, so I think that they should act
like the same key".

As Timothy notes: you *can* do what you're asking for, but it's
reasonably inefficient (from a CPU standpoint, if not a
programmer-enabling one).