  We review the method of uniqueness which is a powerful technique for
multi-loop calculations in higher dimensional theories with conformal symmetry.
We use the method in momentum space and show that it allows a very transparent
evaluation of a two-loop massless propagator Feynman diagram with a non-integer
index on the central line. The result is applied to the computation of the
optical conductivity of graphene at the infra-red Lorentz invariant fixed
point. The effect of counter-terms is analysed. A brief comparison with the
non-relativistic case is included.
