Oops yes, I had forgotten that, PAN is three Xs in morse. That said xxx forms exactly the same keying pattern as PAN. All the radio prowords like MAYDAY and PAN are all post the morse codes, the original distress signal was CQ or “seecue” phontecically, is possibly a shortened form of securite, from the French. The D was added to indicate Danger or Distress. The SOS signal is not really SOS at all as there are no gaps between the letters it’s just an easy way to remember the 9 digit code. It can also be expressed in other ways but SOS is easy.