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
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
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.