[Xy-pic] double arrows

Davide Sangiorgi Davide.Sangiorgi at cs.unibo.it
Tue Mar 18 13:59:27 CET 2014

I'd like to use double arrows in the pictures of a talk.
I tried drawing the double arrows as below, therefore using
\ar at 2@*

\xymatrix at C=1pt{&   *+[]\txt{A}
\ar at 3@*{[|(4)]}[d]
& *+[]\txt{C} \ar@{=>}@*{[|(4)]}[d]
&   *+[]\txt{E}& *+[]\txt{F} \\

In both cases, the distance between the two parallel lines in
the double arrow is thin. Is there any way of making this distance a
bit larger? (Ideally, the same distance as the two external lines in
a 3-arrow  defined using \ar at 3@*)  ?

Many thanks,

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