Sharealike

Sharealike is a required reintegration clause of several parametric licenses in Creative Commons and some monolithic ones like GFDL. It simply means that anyone improving a work must share the improved work with others who wish to use or improve it. It is a key feature of the GPL too which applies this principle to source code. however see problems with free software and open source models.

The CC-by-sa license is quite close to the GFDL but is incompatible with it. Things licensed under the CC regime are not part of the GFDL Corpus.

The CC-nc-sa license is used for GetWiki code and is the closest approximation yet to a Green Documentation License

Reds prefer Free Software apparently because everything "has to be free".

Golds prefer Open Source apparently so they can exploit the commons without obligation to give back.

Trolls like sharing, but bite you on the leg if you try to tell them they have to share with you, even if you are doing harm to them.