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 which 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.