On Mon, Mar 6, 2017, at 18:31, Steve Johnson wrote:
> I can attest to at least one case where AT&T attempted to see whether
> its Unix code had been stolen.  A Unix look-alike came to AT&T's
> attention, and they wanted to get a sense of whether the code had been
> copied before bringing out the big legal guns.   I was one of
> several people asked to log into the system and see what I could
> figure out.  They particularly wanted me to look at their Yacc,
> because they assumed that would be hard to duplicate.
> 
> So I spent an interesting hour checking it out.   The first thing I
> did was to look to see whether some of my bugs or unadvertised
> features were in the program, and they weren't.   Then I threw a
> couple of difficult cases at it, and found a couple of bugs in their
> code.  And besides, it was VERY slow.  So I concluded it was a
> reimplementation.   I gather that was the consensus of others as
> well, and AT&T backed off.

Sounds a lot like Dennis Ritchie's Coherent story:
https://groups.google.com/forum/?hl=en#!topic/alt.folklore.computers/_ZaYeY46eb4