Hello,

I’m solving the Maxwell equations, in Maxwell equations, there is a Gauss law:

so I need to calculate the error regarding the Gauss law, in DOLFINX, I can easily do:

`abs(div(E)-rho)*dx`

but the term E is usually defined in some curl space, so `div(E)`

is not well-posed and I can not calculate it in the strong form, then I would like to calculate the gauss error in the weak form, like:

`abs(E*div(v)+rho*v)*dx`

where v is the test function.

But it seems like the function abs can not be used when I have the test function in the form, and I got the error:

```
raise ArityMismatch("Applying nonlinear operator {0} to expression depending on form argument {1}.".format(o._ufl_class_.__name__, t))
ufl.algorithms.check_arities.ArityMismatch: Applying nonlinear operator Abs to expression depending on form argument v_0.
```

Does anyone know what I could do in this case?

Thanks!