When a program is to be loaded, it is sometimes necessary to
tell the system to treat some of the predicates specially. This
information is supplied by including declarations about such
predicates in the source file, preceding any clauses for the
predicates that they concern. A declaration is written just
as a directive, beginning with :-. A declaration is
effective from its occurrence through the end of file.
Although declarations that affect more than one predicate may be collapsed into a single declaration, the recommended style is to write the declarations for a predicate immediately before its first clause.
Operator declarations are not declarations proper, but rather directives that modify the global table of syntax operators. Operator declarations are executed as they are encountered while loading programs.
The rest of this section details the available forms of predicate declarations.