[texhax] Some puzzling TeX

[Uwe Lueck]

"Philip Taylor (Webmaster, Ret'd)" <P.Taylor at Rhul.Ac.Uk> wrote 20.02.2011 01:40:24:
>Uwe Lueck wrote:
>>
>> "Stephen Hicks"  wrote 17.02.2011 01:50:44:
>>> catcode 16
>>
>> What's that?
>
> See page 209.

Yes, thanks, ... perhaps. 

What evidence is there besides this one of Knuth's notoriously unreliable ("incredible") claims?

I am unable to get a catcode of \relax or \bgroup by \showthe\catcode.
Only with \ifcat, I can see that \relax and \bgroup are different, 
while, e.g., after \let\BGROUP{\et\EGROUP}, \bgroup and \BGROUP 
are the same according to \ifcat.

My conclusion at the moment is that one might better say that 
with \ifcat, control sequences behave "as if they had catcode 16 or ..."
Especially, it seems to me that instead of "catcode 16" one could 
as well speak of "catcode -1" or anything else that is not among 
0, ..., 15.

I am almost a Pascal and truely a C illiterate, 
so can't read the code of TeX, that may allow a more specific statement. 

Let me remark, or repeat for clarification, that as to the original 
question about \@sptoken, Stephen Hicks seems to have reasoned 
about \ifcat only because he had not been told that the "specification" 
of \@sptoken is that it makes LaTeX's \@ifnextchar obey its 
specification, and for this purpose only an \ifx is applied.

Cheers, 

    Uwe.