[Xy-pic] Ensuring arrow labels fit, in xymatrix.

[Scott Morrison]

I know this is a dumb question, but I can't seem to find any mention
in the xyguide.

How do I ensure that column spacing takes into account long arrow
labels? For example, if I have

\xymatrix{1 \ar[r]^{aaaaaaaaaaaaaaa} & 2 \ar[r]^{b} & 3}

the label aaaaaaaaaaaaaaa extends beyond the ends of the arrow. I want
to enlarge the gap between columns 1 and 2, either by end or
automatically depending on the size of the arrow labels.

Similarly; how do I force the row spacing between 2 rows to become
smaller or larger, without affecting the row spacing in the rest of
the matrix?

Thanks, Scott Morrison