[Xy-pic] nested xymatrices

[Russell L. Carter]

Hi,
This is a really easy question I'm sure, but I've put
an embarrassing amount of hours into it, and before
giving up on XY-pic, I thought I'd ask the list.

I would like to group elements together, then
put the group(s) in an xymatrix, and then draw arrows
between the individual elements of the component groups.

For example:

   AB--->CD
         /
        /
       /
      /
     /
   EF   GH
   |     |
    \___/


Squinting really hard at the XY-pic UG and RM, it
seems that I want to start off by doing something like this:

  \xymatrix {
    *{\xymatrix { a & b}} & *{\xymatrix { A & B}}
  }

which doesn't work, nor do obvious permutations of syntax
described in the RM that I tried.  Once I get past that though, I guess
I need to figure out how to individually label each element
of the component groups, and then draw my connectors as
described somewhat in Section 3.6 of the UG.

Any hints would be much obliged.

Thanks,
Russell