!!! Overview
[{$pagename}] distinguished from [copyright], is the practice of offering people the right to freely distribute copies and [derivative work] (modified [versions]) of a [original work] with the stipulation that the same [rights] be preserved in [derivative works] created later.


[{$pagename}] software licenses are considered [protective] or [reciprocal], as contrasted with permissive [Free Software] [licenses]

[{$pagename}] type [licenses] are a novel use of existing [copyright] [law] to ensure a [Original work] remains [freely|Royalty Fee] available. The [GNU General Public License] ([GPL]), originally written by [Richard Stallman], was the first software [{$pagename}] [license] to see extensive use, and continues to dominate in that area. 

[Creative Commons], a [Not-for-profit Entity] founded by [Lawrence Lessig], provides a similar [license]-provision condition called share-alike

!! More Information
There might be more information for this subject on one of the following:
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* [#1] - [Copyleft|Wikipedia:Copyleft|target='_blank'] - based on information obtained 2019-06-04