[Xy-pic] Changing length of arrow in xymatrix

[Adam #]

Hello,

    I would like ask if there is any possibility to change length of arrow
in xymatrix.
    I will show my problem by simple example:

$$\xymatrix at R=1pt at C=1pt{*={a} \ar[rrrr] \ar[dddrrrr] &&&& *={b}\ar[ddd] \\
&&&& \\
&&&& \\
&&&& *={c} }$$

     I am using *={}, because i want to have the smallest diagram, so
without *={} the diagram is to big. The target and source of arrows are
placed in the center of object a,b,c. Is there any option to make the begin
and the end of the arrows in the setting of that symbols ?
I would be very greatful for any sugestions.

Regards
Adam
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