> BTW, using `affix-function' also makes it possible to group completions,
> then the prefix of the first completion in the group could contain
> a group header with group name.

'mule--ucs-names-annotation' has this comment:

  ;; FIXME: It would be much better to add this annotation before rather than
  ;; after the char name, so the annotations are aligned.

With `affix-function' it's possible to add the char before the char name.
Adding groups is also simple (I recall that Richard asked to group by blocks).