SWI-Prolog 9.2.9 Reference Manual: Section E.1

## E.1 Contributing to the SWI-Prolog project

To reach maximal coherence we will, as a rule of thumb, only accept new code that has the Simplified BSD license and existing code with a permissive license such as MIT, Apache, BSD-3, etc. In exceptional cases we may accept code with GPL or LGPL conditions. Such code must be tagged using a license/1 directive (Prolog) or a call to PL_license() for foreign code and, if they are part of the core, the code must be excluded using the `--without-gpl` or `--without-lgpl` option.