Integrating differential forms

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I'd like to integrate dx /\ dy over the unit square. Naively, I would expect the following to work:

E.<x,y> = EuclideanSpace(2) 

phi = E.diff_form(2)
phi[1, 2] = 1

show(integrate(integrate(phi, x, 0, 1), y, 0, 1))

but it fails with:

TypeError: unable to convert 2-form on the Euclidean plane E^2 to a symbolic expression

and I can't find anything about integration in the DiffFormFreeModule documentation. What is the right way to do this?