Hi,

I am looking for a command to calculate the integral of a solved function over a boundary. I tried

```
T_int = 1/V_tot * assemble_scalar(T_fluc * n[0] * ds(1))
```

where V_tot is the volume, T_fluc the solved function, n[0] the normal vector in x direction and ds(1) a specific boundary. In the legacy Fenics, this was possible using the function “assemble(…)”. However, when I try to compute the integral, I get the following error message:

```
TypeError: pack_constants(): incompatible function arguments. The following argument types are supported:
1. (arg0: dolfinx::fem::Form<double>) -> numpy.ndarray[numpy.float64]
2. (arg0: dolfinx::fem::Expression<double>) -> numpy.ndarray[numpy.float64]
3. (arg0: dolfinx::fem::Form<float>) -> numpy.ndarray[numpy.float32]
4. (arg0: dolfinx::fem::Expression<float>) -> numpy.ndarray[numpy.float32]
5. (arg0: dolfinx::fem::Form<std::complex<double> >) -> numpy.ndarray[numpy.complex128]
6. (arg0: dolfinx::fem::Expression<std::complex<double> >) -> numpy.ndarray[numpy.complex128]
```

I am using dolfinx_mpc:v0.4.1.

Thank you for your help!