Unpack and atoms

[f-carraro]

Right now a\a returns an error.

It would be nice if it behaved like newer Ks where it performs base conversion. a/ and a\ would be inverses, for example:

 2/2\1234
1234

I know the same effect can be achieved with l\a, but I think the most use of \ is in this case of equal bases. It would be really useful to have it for atoms instead of an error.