You wrote:
>Let me ask another question: is there a guide to GAP which is
>reasonabely quick to read? The manual is surely extremely complete,
>but I have to confess that in front of the 1,200 pages I am a
>little scared ...

Reading the first chapter should be plenty to get you going. Use the
rest as a reference manual. That's what I did. I don't think the
organization of the manual has changed that much since then, although
it has grown.

I'm not sure about the answer to your other question. I'd have to
check to see whether what I have in mind would work.

David Sibley | "Accurate reckoning. The entrance into knowledge
Amateur radio NT3O | of all existing things and all obscure secrets."
sibley@math.psu.edu | -- The Rhind Papyrus
<URL:http://www.math.psu.edu/sibley/>