After Semweb reads the licensing information, it says that it is licensed under the GPL and Creative Commons Attribution, but that it was originally licensed under the CC Attribution only. It also contains parts that are licensed under the LGPL, and a license for the W3C software. I assume that Rowlex started using Semweb code in the place where it was licensed only in CC, and therefore it did not have to use the GPL.
In addition, I would like to note that there is a rather complicated set of licenses here, and that someone who has more knowledge of all the different licenses can get a better explanation of what is happening.
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