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.