def en_passant_capture
      return nil if self.move.castle

      if !self.board.at(self.move.destination) && self.move.capture
        self.move.destination[0] + self.origin[1]
      end
    end
def en_passant_capture
      return nil if self.move.castle

      if !self.board.at(self.move.destination) && self.move.capture
        self.move.destination[0] + self.origin[1]
      end
    end
If the move is a capture and there is no piece on the
 destination square, it must be an en passant capture.