or Clauses</a>).

<p><code>select</code> then collects all objects which passed through the
filters into a unique list, to avoid duplicates which would otherwise appear,
because most objects can be found by more than one generator clause.

<p>Technically, the filters are normal Pilog clauses, which just happen to be
evaluated in the context of <code>select</code>. Arbitrary Pilog predicates can
be used, though there exist some predicates (e.g. <code><a
href="refI.html#isa/2">isa/2</a></code>, <code><a
href=