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There are  other legitimate free documentation licenses, but sometimes using them requires care. For example, one license is equipped with two optional clauses that can be enabled; the license is free if neither optional clause is used, but enabling either of them makes the book nonfree. (See  http://www.gnu.org/licenses/license-list.html.) Authors that want to publish free documentation, but using a license other than the GNU FDL, can contact us so we can check that the license really qualifies for free documentation.