SWI-Prolog 5.11.18 Reference Manual: Section E.2

## E.2 Contributing to the SWI-Prolog project

To achieve maximal coherence using SWI-Prolog for Free and Non-Free software we advice the use of the LGPL for contributed foreign code and the use of the GPL with SWI-Prolog exception for Prolog code for contributed modules.

As a rule of thumb it is advised to use the above licenses whenever possible and only use a strict GPL compliant license only if the module contains other code under strict GPL compliant licenses.