Thank you for your replies!
I didn't know that there are tabularpackages which are capable
of inserting minimal vertical space between the lines. Maybe one of
these packages (resp. certain concepts) should be built into LaTeX3 to
set a standard and to prevent incompatible packages. (But who decides
which one?)
(With ``arbitrary preambles for the horizontal alignment'' I meant,
that one should have a preamblesyntax just like with the arrayenvironment.)
Now to the problem of \displaystyle in every column. David and Donald
suggested to use \displaystyle as the argument of an ``everymath''command.
This is a solution but it might cause sideeffects if one uses text
inside the array:
\begin{array}{ll}
& \Hom(M,\sum_i V_i)
=& \sum_i\Hom(M,V_i) ,\quad\mbox{since $M$ is $(\bigoplus_i V_i)$projective}
\end{array}
Here an explanation appears inside a calculation. The calculation
should be printed in \displaystyle, but not the `little' mathsymbols in the
explanation. I often use constructions of this form.
Wouldn't it be better to introduce something like ``\everycolumn{}''?
Volker
