[Xy-pic] Dashed curves

[Peter Quince]

Dear colleagues,

I am interested in replacing the curving arrows in the following code

\[\xymatrix{&X\ar@/_2pc/[dd]\ar[d]\ar[dr]\\
&T\ar[r]&0\\
&Z\ar[u]\ar[ur]\ar@/_1pc/[r]&W\ar[u]\ar[ul]}\]

with dashed curving non-arrows.  It seems to me that I should insert a {--}
somewhere after "X" but nothing seems to be working.  I would appreciate any
advice.

Thanks.
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