[plt-scheme] Dependent Contracts for Structures?

Hello all,

Is it possible to create "dependent" contracts (in the spirit of '->d)  
for structs in a provide/contract form?  I'm thinking something like

(struct matrix
   ((rows natural-number/c)
    (cols natural-number/c)
    (elts (vector-of-length/c (* rows cols)))))

When I try this now, I get that 'rows and 'cols are not bound in the  
(vector-of-length/c ...) contract. I don't see anything about this in  
the documentation for the (struct ...) form in (provide/contract ...).

It's not really strictly necessary, but it would be nice to have....

Thanks!
Will
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