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For questions about the copyleft concept (also known as Share-Alike), a concept that promotes or enforces the use of the same or compatible license for derived works.

Copyleft is a licensing principle where the license stipulates that re-distribution of the work or distribution of derivative or combined works is permitted, on the condition that it is done under the same license.

This ensures the work always stays free and/or open.

Well-known examples of such licenses are the gnu family of licenses: the GPL, LGPL and AGPL, the Mozilla public license (MPL), and the Creative Commons licenses with a Share Alike clause, for example CC BY-SA.