bug#3142: 23.0.92; split-window-prefered-function should be able to usesplit-window-horizontally/split-window-vertically

[martin rudalics]

> But if you have a choice between defining foo to be unary or nullary, consider
> defining it with &optional: (defun foo (&optional window)...)
>
> That allows use of foo as an object method for windows. That is, given a 
window,
> `foo' is one of the functions that can be applied to it. If defined as 
nullary,
> this is not possible.

Yes.  But I don't want to burden customizers with the problem in the
first place.  For example, Jared wants to use `split-window-vertically'
as value for this variable and that function has an optional SIZE
argument but no WINDOW argument.

martin