Am 23.07.20 um 14:37 schrieb Michal Hoftich:
> We patch \shipout on one occasion:
>
> \let\:shipout=\shipout
> \def\shipout{\vfil\break\let\shipout=\:shipout \shipout}
>
> If I understand correctly, it is executed only in the first use of
> \shipout and it doesn't seem to do anything useful, although I am
> sure there was a use case at some point for that.
I can't see why it would be of any use. By the time \shipout is called
use are (normally) in the output routine so that the \vfil\break would
end up being placed at the very top of the contribution list and then
later discarded. So I have no idea what the use case may have been
(other than perhaps a misunderstanding of some sort) especially as it,
as you say, only applies to the very first shipout.
> Is this code OK? It should work even in Plain.
Well as far as it goes it should do no harm but in my opinion it could
most certainly be dropped.