bug#8968: arc-mode 7z writing support

[Jason Rumney]

Eli Zaretskii <eliz@gnu.org> writes:

>> The difference between programs `7z' and `7za' is not essential:
>> one of them is just a standalone version of another.
>
> That's not what Jason said.

The non standalone version can load additional external libraries. That
is where the non-Free part comes in.  Aside from that additional
capability, Juri is essentially correct.