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

[Drew Adams]

> Remains the question whether `split-window-preferred-function' should
> always operate on the selected window or have a window 
> argument.  AFAICT
> nothing speaks for the window argument but the fact that 
> people may have
> customized this already.  Omitting the window argument would 
> allow users
> to put `split-window-vertically' directly as value of
> `split-window-preferred-function' without having to delve any deeper.

I haven't followed this thread, and I'm not familiar with the commands in
question.

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.