refL.html#+Link">+Link</a></code> or
<code><a href="refJ.html#+Joint">+Joint</a></code> will be followed, or a
<code><a href="refL.html#+List">+List</a></code> of those will be traversed to
locate appropriate objects.

<li>A list. Then this list should hold a relation and a class (and an optional
hook) which specify some B-Tree <code><a
href="refI.html#+index">+index</a></code> to be traversed to locate appropriate
objects.

</ol>

In this way, a single generator clause can cause the traversal of a tree of
