Thank you for your replies!


I didn't know that there are tabular-packages 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 preamble-syntax just like with the array-environment.)


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 side-effects 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' math-symbols in the
explanation. I often use constructions of this form.
Wouldn't it be better to introduce something like ``\everycolumn{}''?


Volker