r clause is recognized by the fact that its CAR
is a Pilog variable to denote the return value.

<p>The second argument is a list of Pilog variables to communicate values (via
<code><a href="refU.html#unify">unify</a></code>) from the surrounding
<code>select</code> environment.

<p>The third argument is the actual list of clauses for the nested query.

<p>Finally, an arbitrary number of association specifiers may follow, as
described in the <a href="#associations">Indirect Object Associations</a>
section.
