The GNU Free Documentation License (GNU FDL or simply GFDL) is a
copyleft license for free documentation, designed by the
Free Software Foundation (FSF) for the
GNU project. It is the counterpart to the
GNU General Public License that gives readers the same rights to copy, redistribute and modify a work and requires all copies and derivatives to be available under the same license. Copies may also be sold commercially, but if produced in larger quantities (greater than 100) then the original document or source code must be made available to the work's recipient.
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