kingliveson wrote:This is because Rybka's contempt need to be set to zero.

Rybka 4 has no contempt setting (Rybka 3 had). The same effect could be achieved by tuning piece values, but out of the box R4 plays without 'contempt'.

In addition, if a derivative work is sufficiently modified and original to be considered "new", the "new author" and its "new work" then are granted copyright. Knowing Norman's excellent and innovative work, I think it is just fair he is allowed to stamp any license he wants to his engines.

The ...

I do not think so. The original author still holds the copyright and can place any restrictions on the source he wants when he releases it into the public domain...

No, "public domain" means that all IP rights of the author have been forfeited. At least if Wikipedia's article on the topic is to ...

kingliveson wrote:You can't change Public Domain to GPL.

Not true, you can do whatever you want with Public Domain, including modify and release it under different license (GPL, closed source, etc).