This refactoring was already suggested a long time ago, and at the
time both Guido and I didn't like it because it makes the code
actually more complicated: DRY in this case doesn't apply IMO.

Also, this whole thread is a repeat of:
http://bugs.python.org/issue18932

At the time, I already asked for one realistic use case demonstrating
the benefit of implementing modify() natively instead of
unregister()+register().
I know that this code makes modify() faster, but as I said multiple
times, I'd like to se the impact on something else than a
micro-benchmark: arguably if you propose this it's because you've
profiled/found it to be an actual bottleneck, do you have *one*
benchmark to support this change?