>> I agree its purpose is quite different from the example above.
>> Then maybe something like this should do what you want:
>
> Yes, that would be nice if you also think it would be okay to change it
> this way, thank you!

To make sure that everything is right, here is a brief table
for coming changes, where overriden-face is a face specified
by the client:

#+begin_quote
| prefix | suffix | overriden-face | result face             |
|--------+--------+----------------+-------------------------+
|    N   |    Y   |       N        | completions-annotations on suffix
|    Y   |    N   |       N        | no face
|    Y   |    Y   |       N        | no face
|    N   |    Y   |       Y        | overriden-face on suffix
|    Y   |    N   |       Y        | overriden-face on prefix
|    Y   |    Y   |       Y        | overriden-face on prefix and suffix
#+end_quote

Or maybe better to represent this as a test: