[plt-scheme] Signature extension using a-signature

Hello,

Is it possible to extend a signature through a-signature.ss? I think
it is not possible, since a-signature.ss docs do not contain info on
extension but I would like confirmation.

Cheers,

-- 
Paulo Jorge Matos - pocm at soton.ac.uk
http://www.personal.soton.ac.uk/pocm
PhD Student @ ECS
University of Southampton, UK