[Xy-pic] Arrow size

[arijit dey]

Dear Friends,
I am a new user of xy-pic. I heard from my friends
that xypic is a very good package to draw picture in
Latex and I want to learn it.
My first question is how do u control arrow xize
(locally) in commutative diagram of long exact
sequences? 
Whats a good place to learn different aspects of pic?
Regards,
Arijit


       
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