Copyleft is a strategy to encourage sharing or resources. A copyleft measure is copyright so it’s illegal to change it but is released under a licence allowing it to be reproduced unchanged without payment of a fee.
There are a number of different licences designed to allow copyright materials to be reused “copyleft”. The best resource for these is the non-profit Creative Commons organisation who offer a selection of licences. Most creative commons and related “open source” licences for software encourage the copyright owner to full permission for anyone to change the material when reusing it which is generally a bad idea for measures as it loses comparability between results whenever there has been a change. Because of this most copyleft measures are released under “no-derivatives” licenses that prevent changes. This makes translating these measures, such as the CORE measures, illegal without explicit permission from the authors.
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