RE: more than one prefix argument

[Drew Adams]

> Indeed: (prefix-numeric-value nil) -->1
> But does this make sense in a lisp-environement?

Dunno what that means.  nil is not 1.  But (foo nil) might well be 1.  (length
nil) is 0, but that doesn't mean that nil is zero.

`prefix-numeric-value' is just a function that returns a number (always).  If
its arg is nil then it returns 1.  Nothing more to be said about it, really.

> Booleans treat nil and 1 different:
> (when 1 (message "%s" "1"))-->1
> (when nil (message "%s" "1"))-->nil

Yes.  `prefix-numeric-value' is not a Boolean function.  The numeric value of
the prefix arg is just that: a numeric value.  It is a number, never nil, so it
is useless as a (Lisp) Boolean value: it is always non-nil (true).

(Well, you could decide that some particular number or set of numbers
represented false, and then test that way, but you get the idea: it will not
return `nil'.)

The _raw_ value of a prefix arg can be nil, so you can use that as a Lisp
Boolean value (nil vs non-nil).  The raw value distinguishes presence and
absence: whether or not the user specified a prefix argument when invoking the
command.

It also distinguishes other things, however.  The raw arg is richer than its
reduction to a numeric value.  The raw prefix arg tells you pretty much what the
user did - whether s?he hit `C-u C-u C-u' or `C-u 3 2 9' or `M--' etc.

If you want to let the user distinguish several cases using a prefix arg, then
you want to do it using the raw arg.  You can test whether the arg is a cons or
nil or `-'; you can test its numeric value; etc.  And you can test it in
different ways at the same time: both a cons and numeric value = 16, etc.