Re: \hrule issue

Thanks, David, it is clear.  If I set \parskip=0pt then TeX won't stretch that
vertical space at all: the TeXbook says that in plain TeX it is set
\parskip=0pt plus1pt, so if I set \parskip=0pt I'm implicitely setting
`plus0pt' and so forbidding the parskip's stretchability.  But then I wonder
why TeX is allowed to stretch the 1pt enormousely whereas the 0pt not at all.
Suppose I want that a certain \vskip must have a maximum stretchability that
TeX is not allowed to increase: is it possibile to tell TeX not to overcome
anyway a certain amount of stretchability that I decide to set for a certain
\vskip?

Rodolfo