# Integrating differential forms

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?