  We clarify the relationship between the current formalism developed by
Gyulassy, Kaufmann and Wilson and the Wigner function formulation suggested by
Pratt for the 2-particle correlator in Hanbury-Brown Twiss interferometry. When
applied to a hydrodynamical description of the source with a sharp freeze-out
hypersurface, our results remove a slight error in the prescription given by
Makhlin and Sinyukov which has led to confusion in the literature.
