A.3 Defining Relations

The normal method of defining relations in Common Prolog is to use the defrel macro:

(defrel <relation name>
  [(declare declaration*)]
    <clause1>
     .
     .
    <clauseN>)

where each <clause> is of the form:

(<clause-head>
 <subgoal1>
    .
    .
 <subgoalN>)

and declarations may include: (mode arg-mode*) and any of the normal Lisp optimization declarations. Mode declarations determine how much clause indexing will be done on the predicate and can also streamline generated code for a predicate that will only be used in certain ways. A mode declaration consists of the word "MODE" followed by a mode spec for each argument position of the predicate. The possible argument mode specs are:

`?`	Generate completely general code for this arg and don't index on it.
`?*`	Generate completely general code and index.
`+`	Generate code assuming this argument will be bound on entry and index.
`-`	Generate code assuming this argument will be unbound on entry and don't index.

The default mode specs are ?* for the first argument and ? for all the rest.