Do not rely on __sub__ being implemented as __add__ (-1)*
Do not rely on __sub__ being implemented as __add__ (-1)*

There's no point in having two-line comment instead of 6 extra characters and
rely on something that's not a part of the API and may, and in fact did in
Python 3, change.

jkonecny12/blivet,dwlehman/blivet,AdamWill/blivet,vpodzime/blivet,dwlehman/blivet,vojtechtrefny/blivet,AdamWill/blivet,vpodzime/blivet,vojtechtrefny/blivet,rvykydal/blivet,rhinstaller/blivet,jkonecny12/blivet,rvykydal/blivet,rhinstaller/blivet