Jeff Davis wrote:
> Oops, what I meant to say was, when should I use unlock set?
Answer:  If that turned out to have been a mistake, and you don't wind
up moving the set origin in the end.  In that case, you need some way of
getting it unlocked again.

I suppose that an arguably better answer would be for MOVE SET to invoke
LOCK SET at the beginning; succeed or fail, the lock will be gone at the
end of the transaction.  But we didn't go down that particular road...