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I think these should work. Where a piece has a different definition for each side, it will use that for the piece on its own side. The other code for setting last_piece_moved should be used with this. This variable should be initialised in the Pre-Game section, and it should be updated in each Post-Move section. I rewrote None to include the two arguments for origin and destination as other piece functions do. Since it's a contradiction, it will always return false.

  def None == #0 #1 and != #0 #1;
  def White_Joker fn str_replace "Black_" "White_" #last_piece_moved #0 #1;
  def Black_Joker fn str_replace "White_" "Black_" #last_piece_moved #0 #1;