Hey,

Is it possible to calculate the area of the cell faces that are marked in a subdomain?

Below is some test code using a basic hex mesh cube domain. I want to extend this to a complex tetrahedral mesh, so if possible I’d like to avoid iterating through the marked faces and calculating their areas manually.

```
from dolfin import *
from dolfin_adjoint import *
n = 10
Nx, Ny, Nz = n,n,n
mesh = BoxMesh.create(MPI.comm_world, [Point(0,0,0), Point(1,1,1)], [Nx, Ny, Nz],
CellType.Type.hexahedron)
DG_FS = FunctionSpace(mesh, "DG", 0)
CG_FS = FunctionSpace(mesh, "CG", 1)
CG_VFS = VectorFunctionSpace(mesh, "CG", 1)
class top(SubDomain):
def inside(self, x, on_boundary):
return on_boundary and near(x[1], 1.)
class bottom(SubDomain):
def inside(self, x, on_boundary):
return on_boundary and near(x[1], 0)
top = top()
bottom = bottom()
materials = MeshFunction('size_t', mesh, 2)
materials.set_all(0)
top.mark(materials, 1)
bottom.mark(materials, 2)
ds = Measure("ds")(subdomain_data=materials)
# Total area of top
top_area = .... # Exact value is 1 in this case
bottom_area = .... # Exact value is 1 in this case
```

Thanks,

Tom