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[Bartosz Sawicki <sawickib@xxxxxxxxxxxxx>]

I have a problem how to define form for integration over internal facets 
inside VectorElement("Discontinuous Lagrange", "tetrahedron", 0).
Having vector function "j" which is constant over element, I want to 
integrate over all internal facets term dot(j,n), where n is facet 
normal vector.

Consider form:
=====
element = FiniteElement("Discontinuous Lagrange", "tetrahedron", 0)
vectorelement = VectorElement("Discontinuous Lagrange", "tetrahedron", 0)

v = TestFunction(element)
e = TrialFunction(element)
j = Function(vectorelement)
n = FacetNormal("tetrahedron")

a = v*e*dx
L = v*dot(j, n)*dS
=====

Unfortunately it doesn't work. FFC communicates that "Integrand must be 
restricted ('+') or ('-') in an interior facet integral". That is 
confusing for me.

Maybe interior integration operator works differently?
So, would it be possible to use form language to describe such simple 
algorithm?

iterate for every cell c:
  e_i = 0
  integrate for every facet in cell:
    e_i += dot(j,n)
  e(c) = e_i


regrds.
BArtek