== Source == 

 The original source for this seems to be this article﻿: D. Buchholz, Phys. Lett. B 174 (1986) 331. There, the discussion of Gauss's law is extended for the infraparticle case, and this parallels the discussion here. I have not had time to review this source in any detail (it was just linked from the one reference found here), and there might be an earlier one.