ed values successfully pass
<code>select</code>'s filter clauses.


<p><hr>
<h3><a id="db">B-Tree Stepping</a></h3>

<p>In its basic form, a generator clause is equivalent to the <code><a
href="refD.html#db/3">db/3</a></code> predicate, stepping through a single
B-Tree. The clause

<pre>
(nr +Item @Nr)
</pre>

<p>generates the same values as would be produced by a stand-alone Pilog clause

<pre>
(db nr +Item @Nr @Item)
</pre>

<p>as can be seen in the following two calls:

<pre>
: (? (db nr +Item (1 . 4) @I