State Free Objects
I can imagine a Keykos variation with a sort of domain that upon receiving a message would ensure that the domain’s state was the same as any other receipt.
Of course if being reclaimed is part of the domain’s state that takes precedence.
PL/I refers to such a procedure property as REDUCIBLE.
The property should be simply enough defined that a kernel object could vouch for a proffered key as reducible.
It is intended that this property imply discreetness.
There are many problems and it may be impossible.