(BTW I'm sorry I made a .gif and not a .png.  Animation was not strictly
required here, but I don't know the keybindings to make simple
screenshots on this computer.)

>> > What is the purpose of highlighting other parts of
>> > the candidates? 
>> 
>> For a matching style such as flex or substring (as you would find in 
>> many other editors) it's important to visually explain users to users
>> why certain strings that don't start with the pattern they entered 
>> are being shown on the screen.  I attach an image.
>
> I don't see why it's important to explain how did the completion
> algorithm arrive at a particular candidate.  The completion algorithm
> is there to intuit what we mean in the most efficient way, but the
> details of how it does that are immaterial.  The only ones who may be
> interested are those who study completion algorithms ;-)

I may sound like a completion scholar to you, but you also sound like
you haven't experimented with 'flex' for more than 1 second, otherwise
you wouldn't be asking that question ;-) .  So I attach another Emacs -Q
gif, crippled-flex.gif, where you see the current problem, and yet
another gif, useful-flex.gif, where the matching part is highlighted
bold.