Re: @vskip ...

[Karl Berry]

    1) Knuth does not encourage the use of filll (I don't pretend to
       understand why), 

The idea, as Knuth vaguely implied, is that it's good to "normally" use
fil and fill so that filll can be used in an "emergency" to override
them. E.g., if someone is in the last stages of producing a book and
needs to force the spacing.

       why does Texinfo has use only for filll and not
       for the 2 others ?

The whole filll usage in Texinfo is a kludge that has been there since
day one, solely for purposes of the copyright page, where it's
conventional for the material to be typeset at the bottom. 10 minutes of
thought to handle that convention differently in Texinfo input back in
1985 and it would never have been needed.

    2) Texinfo seems to use filll exclusively in the context of the
       copyright page, 

Right.

       so why not create a Texinfo "primitive" that would hide the
       complexity of the "incantation", something like @vfilll ?

Precisely because it is only used on the copyright page, I see no use
for such a command. It makes no sense in other output formats.

What could make sense: since the title page is already specified with
@titlepage...@end titlepage, a new command like @titlepagereverse could
be defined, which would do both the

@page
@vskip 0pt plus 1filll

and then @vskip would not be needed or wanted anywhere. Whether doing
this or something similar is worth bothering at this point (since @vskip
would have to stay defined forever since essentially all existing
Texinfo documents use it), is up to Gavin.

    @deffn Command vfilll
    vskip Opt plus 1filll
    @end deffn

That is documenting a command named "vfilll" (like one might document a
C function in a manual). It has nothing to do with defining a new
Texinfo command. For that you need @macro, at the document level. In
texinfo.tex, the standard TeX syntax can be used:
\def\vfilll{\vskip 0pt plus1filll\relax}

    3) Is it OK to add a reference to that part of the TeXbook in the
       Texinfo manual so that users understand why this thing exists ?

The TeXbook is, sadly, nonfree documentation, so it's not ideal to refer
to it. I don't know that it's totally forbidden, but in this case, there
are at least two other free books that also describe it (and everything
else in primitive+plain TeX: TeX by Topic and TeX for the Impatient.

However, I admit I don't see the need for such a reference. The Texinfo
manual already explains everything that Texinfo authors need to know
about it, seems to me. --best, karl.