Refactor

I’m going to keep this one quick because it just got deleted for being too long.

The previously mentioned behaviour of overwriting filenames has been fixed and has it’s own unit test now. It was given an exception before the actual rename.

All the rename logic was duplicated between dec and !dec, so I flattened that.

I pulled out the pre-rename exception logic into a new helper: skip_rename(). It needs unit tests of its own.

Byyyeeeeee!!!