Chris Rowley writes:
 > Frank, Lars
 >
 > Take care!
 >
 > I have not followed all the details of your debate but I get the impression
 > that you may be wrong about how \vspace interacts with \addvspace.

oops, indeed. but fortunately it doesn't actually make that particular problem
go away it only makes more prominent, eg

\addvspace{8pt}
\addvspace{10pt}

gives 10pt of space while

\addvspace{8pt}
\vspace{2pt}
\addvspace{10pt}

give 20pt not 18pt as i claimed. and it is still not clear for the user what
value to use in \vspace to get say 2pt more space without knowing the actual
values in the \addvspace commands that have been used internally.

so i'm would not call this:

 > This is best described as `no interaction', since \vspace and
 > relatives all `end with'
 >
 >   \vskip\z@skip .

it is more "kill interaction" and that was the problem i pointed out.

thus i would say its purpose is not so clear rather than:
 >
 > The purpose of this is that, in LaTeX, _only_ those spaces
 > added by \addvspace should interact (and with \addpenalty);
 > all other spaces are irrelevant.

since an extra space via \vspace is fairly "relevant"

right?

frank