You don't change licensing. You got choice of two licenses, and you
selected one. GPL gives you right to remove the BSD licensing terms
you did not choose.
(People still can use the code under BSD license _if_ they know it was
originally BSD/GPL.. as long as you don't modify it).
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(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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