Unfortunately, that's not so easy. You've created a derived work, which can't simply claim full copyright for.
In the realm of legal battles, wording matters. Claiming copyright is ok as long as there was "substantial work" put into creating the Derived Work. Claiming the right to choose a lincense is quite another matter and depends on the license of the Original Work.
It turns out that the file mentions multiple authors. Now the question is: Who is the copyright owner
Each one is a copyright owner, and perhaps *all* would have to agree on a license change, depending on the license(s) the file had through its history. It's also a matter of courtesy.
that can give us the right to use, modify and redistribute that file? Pierre A. MacKay seems to be the original author. H. Turgut Uyar has modified it. Now, who owns the copyright?
It depends. For a start, Mr. MacKay apparently didn't create his stuff for the sole purpose of creating hyphenation files. This means the creator of the TeX file can not only claim copyright but also created an Original Work rather than a Derived Work, so he can also choose a license, as long as the original doesn't have a very restrictive license (which would probably also make creating and using the TeX hyphenation file illegal, so I suppose this isn't the case). There is still the problem that the TeX file is explicitely labeled as "Mechanically generated", which can be interpreted in a way that there wasn't "substantial work" put into creating the TeX file, which means it isn't even a Derived work but basically another representation of the original material and Mr. Uyar can't even claim copyright. The "slight modifications" however should be enough to fix this. Therefore, depending on the interpretation we've either have to ask Mr. Uyar only (interpretation 1) or both (interpretation 2).
http://www.ctan.org/tex-archive/language/turkish/hyphen/turk_hyf.c from Pierre A. MacKay which states that in case of commercial distribution the author must be contacted.
This is not a real problem as long as the license doens't require that Derived Works must be distributed under a similar condition.
that there could be problems with licensing. The whole TeX thing is a legal nightmare if you ask me.
Well, the whole Intellectual Property stuff is a nightmare. This is a direct result of assigning substantial value to intangibles...
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