For each built-in predicate a subclass of Pred is needed. Each
built-in predicate must be introduced in BuiltIns class.
All data and programs are represented as Pro_Term objects. Every
Pro_Term object has a pointer to a Pro_TermData object
that contains the associated data.
If the only purpose is to add new built-in predicates the only source files
needed are BuiltIns.java and Version.java.
These source files can be extracted from the jar file jalog.jar:
jar -xf jalog.jar src/io/github/JalogTeam/jalog/BuiltIns.java
jar -xf jalog.jar src/io/github/JalogTeam/jalog/Version.java
The complete source package can be found at https://github.com/JalogTeam/Jalog.