> By error prone, it mean that it's easy to introduce a bug or a regression,
> since the code is complex and almost nobody maintains it.

Indeed. Actually, I encountered some faults when I migrated original
patch. The character encoding is a kind of specialty area. This patch
is written by Masayuki Moriyama, who is an expert of character
encoding and he have been contributed to various communities for a
long time. Also, he helps me to migrate original patch(for Python
2.4.3) to Python 3.5. You can see commit log he fixed some bugs.
https://bitbucket.org/t2y/cpython/commits/all

> I'm not stongly opposed to any change. I'm just trying to understand the
> context.

Thanks. I'll help it by explaining the context.