“…In the meantime we learnt from the literature that appeared since 2003 (see, e.g., Delphenich [8,9], Itin [24], Kaiser [26], Kiehn [29], and Lindell & Sihvola [35,37]) and improved our derivation of the light cone, simplified it, made it more transparent (see, e.g., [21,23,33,45,46]). The formalism and the conventions we take from [20].…”