[Xy-pic] smaller arrows

[Baptiste Calmès]

Hi,

My question is about making arrows smaller, i.e. starting further  
away from their source (and arriving not too close to their target)?  
My problem appears when I am grouping objects, using bracket frames  
arround them, and then adding an arrow from one group to another. The  
arrow overlaps on the bracket where it starts.

I have seen some messages about that by searching the archives of the  
mailing list, but I'm not sure I understand how it works. For  
example, in this line:

\POS "1,4",\ar@{}"3,5",\ar"3,5"!E-/15pt/*{}

How should the
!E-/15pt/
be decomposed/understood?

I can see its effect, but I don' t really understand how to modify it  
to change it slightly. What does the E stand for, for example?

Thanks for any help,

Baptiste