Integration over lines (entities of codimension 2)

Hello everybody!

I have seen that using dx and ds (and dx(i), ds(i), suitably marking the mesh via MeshfFunction) it is possible to integrate over entities of codimension 0 and 1, respectively.
I wonder if there is a way of integrating over entities of codimension 2 (set of points in 2D, lines in 3D).

I know that it is possible to mark lines, and even points, via EdgeFunction and VertexFunction, but it seems to me that there is no chance to pass this information to a Form, since only
Form::set_cell_domains (codimension 0)
Form::set_exterior_facet_domains (codimension 1)
Form::set_interior_facet_domains (codimension 1)
are available.

The following (C++) 2D code snippet might be useful to explain what I mean.

class TopBoundary : public dolfin::SubDomain
{ 
    bool inside (const dolfin::Array<double>& x, bool on_boundary) const
    { return on_boundary && dolfin::near(x[1],0); }
};

class ExtremaOfTopBoundary: public dolfin::SubDomain
{
    bool inside (const dolfin::Array<double>& x, bool on_boundary) const
    { return on_boundary && dolfin::near(x[1],0) 
        && ( dolfin::near(x[0],0) || dolfin::near(x[0],1) ); }
};

int main(int argc, char* argv[])
{
    dolfin::UnitSquareMesh mesh(10,10);

    // working (codimension 1)
    dolfin::EdgeFunction<std::size_t> boundaryFunction (mesh);
    boundaryFunction.set_all(0);
    TopBoundary topBoundary;
    topBoundary.mark(boundaryFunction,1);
    elsewhereDefinedForm.set_exterior_facet_domains(boundaryFunction);
            // employable in a form via ds(1)

    // aim of the question (codimension 2)
    dolfin::VertexFunction<std::size_t> pointsFunction (mesh);
    pointsFunction.set_all(0);
    ExtremaOfTopBoundary extrema;
    extrema.mark(pointsFunction,2);
    elsewhereDefinedForm.WHAT_TO_PUT_HERE(pointsFunction);
            // longing to be employed in a form via something like ds(2)

    // some other stuff
    return 0;
}

Thank you in advance,

Ivan