[Xy-pic] **{} and !E

[Witek Kwiatkowski]

Hi all,
I am very new to xy-pic (Nota bene phenomenal software!!!) and  I have 
difficulties to understand how it handle positions.
Here is the example I do not understand.
\xy
<1cm,0cm>:
(1,1) *=<50pt>{a}*\frm{-}; (2,8)**{}, (3,5) *=<70pt>!E{c}*\frm{-}
**\dir{-}
\endxy

According to the manual !E should move a reference point to the edge 
point which is lying on the line from p (2,8) to the center of {c}.
Also from the manual <pos>, <coord> will move c to (3,5)  and **{} will 
connect (1,1) with (2,8) without affecting positions.
However in this case **{} and !E are related. What seems to be going on 
here is that **{} is cuting the box of {a} in two points one in in the 
direction of center of {a} to the current position (2,8) and the other 
is the opposite. And this opposite transfered in proportional relation 
to  box {c} will become a new reference point after !E.
so for example if we put p!L instead of (2,8)  !E will correspond to !R  
on {c}.
Could someone point out the mechanism of such behaviour.
Thanks,
Witek