SWI-Prolog 7.1.16 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 advise using LGPL for contributed foreign code and using GPL with the 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 use a strict GPL compliant license only if the module contains other code under strict GPL compliant licenses.