I don't think we need to get into the theoretical weeds here.  Unless
there are objections, here's the solution I'll adopt.

It is an error to call fxmapping-update-min / -max or
fxmapping-pop-min / -max on an empty fxmapping.  Since it's probably
trivial to check for an empty fxmapping, providing an optional failure
continuation here is overkill.  So we get rid of the 'failure'
arguments in these cases.

fxmapping-update is internally consistent, and doesn't need a
failure continuation--if you want that, you use fxmapping-alter.

--
Wolfgang Corcoran-Mathe  <xxxxxx@sigwinch.xyz>

"This isn't right. This isn't even wrong." --Wolfgang Pauli