At 00.09 +0100 2003-03-07, Frank Mittelbach wrote:
>i starred at that definition once more (probably done so several times in the
>past :-) but i think it has some understandable logic (i'm not saying that it
>has the world's best implementation for it) after all.
>
>basically it goes like this:
>
>the main intention why the command was introduced in the first place to is the
>following:
>
>  if there are two objects A and B that both want to
>  produce some space after/before them  then if both objects are directly
>  after each other one usually does not want to put too much space between
>  them, so a good approach is to use the maximum

Yes, that makes perfect sense (although the name does not).

>now an assumption that i think was made is that the skips that
>actually end up being on the galley are positive and in fact i
>think this is virtually always the the case. Note that i talk
>about the skips on the galley not the skip used the
>last time in an \addvspace command, e.g. the cited example
>
>        \addvspace{-\parskip}   %% added 4 Sep 85
>
>i actually one in a sequence
>
>        \addpenalty\@beginparpenalty
>        \addvspace\@topsep
>        \addvspace{-\parskip}%
>
>and the net effect of \topsep-\parskip is normally positive.

Yes, but then it should have been

    \addpenalty\@beginparpenalty
    \@tempskipb=\@topskip
    \advance \@tempskipb -\parskip
    \addvspace\@tempskipb

because otherwise the net results will be wrong (less than \lastskip) when

    \topsep-\parskip < \lastskip < \topsep


>so back to the theme if producing the maximum is sensible then what to do
>about
>
>    SKIP < 0pt ?
>
>i think that Leslie thought this to be a good way to extend the command to
>allow for corrective actions if necessary
>
>why he additionally required  \lastskip > 0pt is beyond me though.
>
>it means that  SKIP < \lastskip < 0pt again uses the maximum principle but
>that doesn't make much sense to me.
>
>so much for how i read history two minutes before midnight (perhaps all
>rubbish)
>
>well, how could a simpler command look like?

A first approximation would be like this:

\def\MinimalVspace#1{%
   \ifvmode
      \if@minipage \else
         \ifdim \lastskip =\z@
            \vskip #1\relax
         \else
            \@tempskipb#1\relax
            \ifdim \lastskip<\@tempskipb
               \vskip-\lastskip
               \vskip\@tempskip
            \fi
         \fi
      \fi
   \else
      \@noitemerr
   \fi
}

It is quite possible that the special code for \lastskip=\z@ should also be
removed, but with \parskip = 0pt plus 1pt being quite common, that change
could actually be noticable.

As for the name, I think this would be more intuitive. The user (or perhaps
rather class designer) is requesting that at least this amount of vspace
should appear at this point.

>>Also, it seems to me as if there could be a need for two commands here: one
>>which does what (the simplified) \addvspace does (guarantees that there is
>>at least a certain amount of vertical space in the last mix of penalties
>>and glue), and one which actually makes the vertical space a certain amount
>>larger (without messing it up for any subsequent \addvspace or
>>\addvpenalty).
>
>i can see how to define a user command for corrective actions always adjusts
>the space in either direction, the only problem is that its natural name
>(addvspace) is already taken

Well, there's always

\def\AddVspace#1{% Or maybe \reallyaddvspace
   \ifvmode
      \if@minipage \else
         \@tempskipb#1\relax
         \advance \@tempskipb \lastskip
         \vskip-\lastskip
         \vskip\@tempskip
      \fi
   \else
      \@noitemerr
   \fi
}

I'm not too certain about whether the minipage condition should be present
here, though.

Lars Hellström