Thank you Mark and Eric.

> I'll note that the paper is proposing a 'z' modifier to the sign, so I guess for us that would translate to: [sign[optional-z]] instead of just sign.  I'd have to noodle through the differences between that the proposed [sign][~].

The C++ paper proposes [sign][z] (i.e. you can have the `z` alone without an explicit +/-), and this is what I implemented in the Python PR.  My original proposal with tilde was discarded.

> My only reservation is Mark's comment:  """For Decimal, we'd need to "own" the string formatting, taking that responsibility away from mpdecimal, but there are already other reasons to do that."""

In the PR I was able to avoid taking that on by preprocessing the format string before handing it to mpdecimal.  The code was already doing such things to handle the NULL fill character.