One of the reasons that Mark Williams attracted the attention of AT&T lawyers was because they actually engaged former members of the Bell Labs UNIX research group
who had prior access to ATT research source code to work on pieces of their system.
THE BIG RED FLAG....

However, everyone at MW was painfully aware of the IP lawsuit potential in what they were doing. So they took great pains to avoid that occuring.

I believe they could read the MAN page and any supporting documents, But they had to write everything from scratch.

This is from a USENIX conference dialog with a mutual friend who passed through MW on his way west to fame and fortune.

I believe a version of Coherent resides at Sourceforge in the operating systems archives.

On Sun, May 1, 2022, 8:01 AM Rob Pike <robpike@gmail.com> wrote:
The folks at Bell Labs were asked to figure out if Mark Williams had copied Unix directly or via too much knowledge already obtained, or whether it was truly a clean room recreation. I don't remember all the details, but it became clear after a while that it was indeed a reasonably clean rewrite.

This was done by looking for corner cases that were an accident of the original implementation and would be unlikely to appear in a version created separately. One detail that did stick with me was the discovery during this process that ppt, the paper tape simulator, mispunched a letter, I think "R", but the Mark Williams version did not. Was that compelling? Not on its own, but it was funny and memorable.

-rob


On Sun, May 1, 2022 at 9:46 PM Ron Natalie <ron@ronnatalie.com> wrote:
Mark Williams Coherent was one I worked with on the PC many years ago.

> On May 1, 2022, at 11:34, Andrew Warkentin <andreww591@gmail.com> wrote:
>
> What was the first "clone" functional Unix (i.e. an OS not derived
> from genetic Unix code but highly compatible with genetic Unix)? Idris
> is the earliest such OS of which I am aware (at least AFAIK it's not a
> genetic Unix), but was it actually the first? Similarly, which was the
> first "outer Unix-like" system (i.e. one with strong Unix influence
> but significantly incompatible with functional Unix)? Off the top of
> my head the earliest such system I can think of is Thoth (which
> predates Idris by almost 2 years), but again I'm not sure if it was
> actually the first.