This is almost the same as
(target-element-list), except that it lists those elements
which are an allowable target of a REF element. The difference is
that when we link to a P, the target of the REF is the P element,
but the link that is generated is to the first ancestor of the P
which is within (target-element-list). If there isn't
this distinction, we get a processing loop.
Type: list of strings
List of ref targets