> Actually, I discovered that unfortunately annotation-function
> is unsuitable for formatting data in a columnar format.
> It needs to receive a complete list of completions to be able
> to adjust their widths for more optimal overall columnar layout.
>
> So a new function is needed that is like annotation-function
> but accepts a list of all completions.  There is an existing
> similar function 'display-sort-function' that could be abused
> to add prefix/suffix annotations, but maybe a separate function
> would be preferable for cleaner API?

It turned out that 'display-function' is not a suitable name either.
What a new function should do is to prepend/append affixes
on both sides of a completion, accept a list of all completions,
and return a list of two separate strings for prefix and suffix: