I should be able to change the creatorship / ownership of a Thing
        
          Summary: Requested by Eric, as part of managing his Space.  He makes use of _creator for a bunch of stuff, but that raises a problem: how do you change that?  How do you reassign the "ownership" of a Thing?  It's a reasonable desire.
The implication here is that we have overloaded the concept of "creator": it means what it means, but it also means "owner" in a very real sense.  That relationship should be weaker.
Offhand, the best approach is probably to add _owner as a separate but closely related idea.  This would inherit from _creator, and would usually be the same value.  But unlike _creator, it makes semantic sense to be able to change this value with a function.