Are any parts of UNIX in public domain?

[Jeff Siegal]

In article <339 at brl-smoke.ARPA> gwyn at brl.ARPA writes:
>It's REALLY funny to put a copyright notice on an empty file; does that
>mean that all those empty files you have lying around are illegal copies
>of AT&T's /bin/true with the copyright notice removed?

Or better yet, does it mean that all code which is a superset of that
empty file is an extended version of AT&T's code?

Jeff Siegal