The compiler usually generates an index on the principal functor of the first argument of a predicate. The user may direct the compiler to generate an index on any other argument by means of an indexing directive. This is of the form
argument of the predicate Pred/Arity.
All of the values Pred, Arity and IndexArg
should be bound in the directive:
Pred should be an atom, Arity a nonnegative integer, and
IndexArg an integer between 0 and Arity.
If IndexArg is 0, then no index is created for that predicate.
As an example, if we wished to create an index on the third argument of a 5-ary
predicate foo, the compiler directive would be