Cleanups to ax-bnd comment in iset.mm.
Cleanups to ax-bnd comment in iset.mm.

The worst one is giving the wrong variable name in explaining the axiom.

There's also a typo (read→reason) and also trying to be a bit more
accurate when talking about decidability in the context of an axiom
which isn't (quite) stated in terms of decidability.

metamath/set.mm,metamath/set.mm,metamath/set.mm