cifies some kind of database B-Tree <code><a
href="refI.html#+index">+index</a></code>, to be traversed by
<code>select</code>, step by step, where each step returns a suitable single
database object. In the simplest case, they consist like here just of a relation
name (e.g. <code>nr</code>), a class (e.g. <code>+Item</code>), an optional hook
specifier (not in this example), and a pattern (values or ranges, e.g. (1 . 4)
or "Part").

<p>The generator clauses are the core of 'select'. In some way, they behav