>>183
You make a good point about needing a medium-agnostic definition of "being required to share source."
I agree that an ideal copyleft/share-alike license ought to allow for re-licensing under other complaint copyleft/share-alike licenses. However, re-licensing can get pretty hairy. The Creative Commons share-alike licenses, for example, limit re-licensing to other Creative Commons share-alike licenses. I don't know exactly why they limited re-licensing in this way, but it's probably just a case of reducing exposure and potential loopholes. They have a lot more legal counsel at their disposal than I, so. ¯\_(ツ)_/¯
I don't think an "or stricter requirements" phrase would work since that would appear to include proprietary licenses. To describe what licenses are compatible, you'd probably need to enumerate a legal definition of the requirements of copyleft and the Four Freedoms of Open Source Software.
parts of the GPL are pragmatic
Yeah, I feel like I'm fighting against the complexity of trying to translate philosophies into legal definitions.