P410diff:
  exists from 21.42 to 81.42 degrees in the iasp91 model.
  with degenerate ray parameter of   11.521 sec/deg.
  travel times from   292.39 to   983.68 sec.

  P410diff going down as a P in the crust/mantle, layer 0 to 3, depths 0.0 to 410.0, then diffract
  P410diff going flat as a P in the mantle, layer 3, depths  at 410.0 (DIFF), then turn after diffract
  P410diff going up   as a P in the crust/mantle, layer 3 to 0, depths 410.0 to 0.0, then end

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