Since version 2.1.6, Discount has been released under the 3-Clause BSD License which [is GPL compatible](https://www.gnu.org/licenses/license-list.en.html#ModifiedBSD)
[2.1.6 release notes](http://www.pell.portland.or.us/~orc/Code/discount/#v2.1.6)
[Current Discount COPYRIGHT file](https://github.com/Orc/discount/blob/master/COPYRIGHT)