FSF distinguishes additionally between free software licenses that are compatible or incompatible with the FSF license of choice, the copyleft GNU General Public License. The Open Source Initiative defines a list of certified open-source licenses following their The Open Source Definition. Also the Debian project has a list of licenses which follow their Debian Free Software Guidelines. Free and open-source licenses are commonly classified into two categories: Those with the aim to have minimal requirements about how the software can be redistributed (permissive licenses), and the protective share-alike (copyleft Licenses). An example of a copyleft free software license is the often used GNU General Public License (GPL), also the first copyleft license. This license is aimed at giving and protecting all users unlimited freedom to use, study, and privately modify the software, and if the user adheres to the terms and conditions of the GPL, freedom to redistribute the software or any modifications to it. For instance, any modifications made and redistributed by the end-user must include the source code for these, and the license of any derivative work must not put any additional restrictions beyond what the GPL allows. Examples of permissive free software licenses are the BSD license and the MIT license, which give unlimited permission to use, study, and privately modify the software, and includes only minimal requirements on redistribution. This gives a user the permission to take the code and use it as part of closed-source software or software released under a proprietary software license. It was under debate some time if public domain software and public domain-like licenses can be considered as a kind of FOSS license. Around 2004 lawyer Lawrence Rosen argued in the essay "Why the public domain isn't a license" software could not truly be waived into public domain and cannot therefore be interpreted as very permissive FOSS license, a position which faced opposition by Daniel J. Bernstein and others. In 2012 the dispute was finally resolved when Rosen accepted the CC0 as an open source license, while admitting that contrary to his previous claims, copyright can be waived away, backed by Ninth circuit decisions. See also Comparison of free and open-source software licenses Digital rights management Copy protection Dual-licensing Index of Articles Relating to Terms of Service and Privacy Policies License-free software License manager Product activation Product key Rights Expression Language Software metering Terms of service Perpetual access Copyright licenses (category)