Kernel structure and qualifications

For $m=0$, the flat kernel satisfies Huygens' principle, whereas nonzero diffraction produces a wake beyond the geometric fronts. This does not mean propagation outside the intrinsic causal relation. With nontrivial flux, diffraction can persist even at an untwisted pure-image cone; it disappears there only for the compatible characters in (eq:pure-images) AharonovBohm,Dowker1987.