> Similarly, I see that
>
>     (completion-pcm--merge-completions '("abr" "absabr") '(prefix "br"))
>
> returns
>
>     ("br" prefix)
>
> whereas it should arguably return
>
>     ("br" "a" prefix)
>
> [ Tho this may have the side effect that after this completion, `absabr`
>   won't be considered any more, if the `basic` completion comes before
>   `substring` :-(  ]

I did notice this too.  I could try fixing/changing this too, but it
does seem annoying when basic comes before substring - as it does by
default in a number of completion categories.

I wonder if we should move basic to after substring in those categories
in completion-category-defaults?  Or just remove basic from them.  It
doesn't seem like having both basic and substring in those lists has
much point.  Plus it would be a nice improvement to defaults - buffer
completion in particular confused me for a long time before I understood
how basic and substring interacted, so having only substring would ease
understanding for new users.