bug#6459: 24.0.50; completions-format should not have a `nil' choice

[Drew Adams]

> >> (defcustom completions-format nil
> >>   "Define the appearance and sorting of completions.
> >> If the value is `vertical', display completions sorted vertically
> >> in columns in the *Completions* buffer.
> >> If the value is `horizontal' or nil, display completions sorted
> >> horizontally in alphabetical order, rather than down the screen."
> >>   :type '(choice (const nil) (const horizontal) (const vertical))
> >>   :group 'minibuffer
> >>   :version "23.2")
> >>
> >> There is no reason for the `nil' choice.  Please remove it 
> >> and make the default value `horizontal'.  Occam's razor
> >> (KISS) and clarity for users argue for this.
> >
> > We could remove nil from the docstring and from :type, and change
> > the default value to `horizontal', but I'd still treat nil as
> > `horizontal' internally for compatibility.

I don't have a big problem with that. But what "compatibility" is gained?
Compatibility of what with what? This is a new variable.
 
> Since this is a new defcustom woulddn't it be better to not let nil be
> a valid value? That may catch some problems and the backward
> compatibilities are not likely to be big.

I agree with Lennart here. KISS. I see no reason for nil. I am unaware of any
backward compatibility issue for this variable.