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[Alessio Quaglino <alessio@xxxxxx>]

There is a function in FFC which seems very useful for my aim but I don't understand how to use it.  One part of my form is the following:

n = FacetNormal("triangle") 
dsD = Integral("exterior facet", 0) 
dsN = Integral("exterior facet", 1) 
dsF = Integral("exterior facet", 2) 

a = ( dot(psi,b) - div(psi)*w + phi*div(b) )*dx + ( dot(b,n)*psi[0] + dot(b,n)*psi[1] )*dsF
L = phi*f*dx - dot(psi,g)*dsN

What I want to do is to assemble the matrix splitting the boundary in 3 parts: dsD (no terms), dsN (1 term in a), dsF (1 term in L). However I don't understand how to pass to the form the subdomains I defined in dolfin. For example I have:

class DirichletBoundary : public SubDomain
{
  bool inside(const double* x, bool on_boundary) const
  {
    return x[0] < DOLFIN_EPS;
  }
};

which I would like to associate to dsD. Is there a way to do that?

Thank you,
Alessio