A function rule is an item of the form
‘Head = Result :- Body’.
A predicate rule is an item of the form
‘Head :- Body’
where the top-level functor of ‘Head’ is not =/2.
In both cases, the Head term must not be a variable.
The top-level functor of the Head determines
which predicate or function the clause belongs to;
the predicate or function must have been declared
in a preceding ‘pred’ or ‘func’ declaration in this module.
The Result and the arguments of the Head
must be valid data-terms
(optionally annotated with a mode qualifier;
see Different clauses for different modes).
The Body must be a valid goal.