I am not sure this is the place to post this but...
In com.ugos.jiprolog.engine.Retract1.java at line 72 the line
en = ((DefaultClausesDatabase)db).clauses(functor);
should probably be
en = ((JIPClausesDatabase)db).clauses(functor);
to allow a user created in-memory data base to implement their own store structure other than the Vector that is part of DefaultClausesDatabase.
It has been fixed in the current version on github