I hit the 'post' button too soon :-/

> And I would be happier if my silly string-literal example produced a fatal
> error! Otherwise, I can't imagine a scenario where a 'naked' string would even
> be used there.
>

Well, on 2nd thought, I do see at least one special-case string use: To try to
easily determine if a particular #include file is actually present in a scene.
Like:
              #ifdef("my sphere file.inc")
or maybe a no-string version:  #ifdef(my sphere file.inc)
                           or  #ifdef(my sphere file)

Nothing like this works in v3.8xx. Of course, #ifdef may not be the correct
thing to use, but it's the first idea that comes to mind. (I have a vague
memory-- possibly mistaken-- that I did use the string- construct version in the
*distant* past, and that it worked. Or maybe not.) But with the current behavior
of #ifdef("...string..."), it's of no use. And   (my sphere file.inc)   fails
outright because of the 'dot'; likewise   (my sphere file)  because of the
spaces.

There IS an obscure built-in identifier (or variable?) called
View_Include_Stack, that can be used to determine if any of POV-ray's *own*
specific include files are present-- ALL that are being used-- by putting the
following at the top of a scene, running it, then looking afterward in
'Messages' (in Windows GUI versions, that is):
                     #declare View_POV_Include_Stack = true;

I can't find it in the documentation; but take a look here...
https://news.povray.org/povray.general/thread/%3C5c0827cd%241%40news.povray.org%3E/

Unfortunately, it will not list any user-made #includes-- even if they are
placed in POV-ray's 'includes' folder; I just tested it. That seems strange.

(The link above actually discusses the topic of how to determine the presence of
such user-made files, but the various methods seem to require that code snippets
be added to each and every file first. A simpler #ifdef scheme would be much
easier!)

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