+facet facet string For a `facet` widget, this setting is mandatory, and indicates which field to
+ list terms for. Three fields are supported: `subject`, `author` and
+ `xtargets` -- the latter a special case which treats the target providing a
+ record as a facet. Any other field may also be used, but the default caption
+ and maximum term-count may not be appropriate, needing to be overridden by
+ `facet_caption_*` and `facet_max_*` settings.