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I have removed the mention of free/open source, since it doesn't look like this is the case. It seems that Chipmunk has some severe restrictions (based on the income of the user, no less!) on its "pro" version, and I don't think that it is appropriate to support the illusion that it is completely free. It's probably possible to edit this and specify which parts are open source and which parts are not, but I personally feel that even that would be dishonest if it doesn't mention the full version's license.