License info request

[Keith Willis]

Hi,

I am in the process of putting together a source code package for a
library we use.  The source code consists of a number of different
modules which are all labelled in one of the following ways:

1. Code written by me which I have labelled as being under the GPL.

2. Code written by Alice and modified by me which is labelled by Alice
as being under the GPL.

3. Code written by Bob and modified by me which is labelled by Bob as
being Public Domain.

4. Code written by Carol and modified by me which is labelled as
"Licence is also granted to make and use derivative works provided that
such works are identified as 'derived works from the RSA Data Security
(Oops, I mean Carol!), Inc BARF-Jiggle Algorithm' in all material
mentioning or referencing the derived work."

I guess my question is am I in some way 'diluting' the GPL status of my
own code and mods to other GPLd code by including PD and 'free but
proprietary' code in my library package?

I _think_ that in case 3, I can put a GPL notice on the code, stating
that my modified version of this code is henceforth subject to the GPL,
and that the original code was in the public domain and can obtained
from Bob's web-wite.

I have no idea about case 4.

Oh, and if there's an FAQ about GPL issues such as this, I'd appreciate
a pointer in the right direction.

Thanks.