code: def ref(host, seq, takeoff, emergency=False):
      
      p = 0b10001010101000000000000000000
      if takeoff:
          p |= 0b1000000000
      if emergency:
          p |= 0b100000000
      at(host, 'REF', seq, [p])
docs: Basic behaviour of the drone: take-off/landing, emergency stop/reset)
  
      Parameters:
      seq -- sequence number
      takeoff -- True: Takeoff / False: Land
      emergency -- True: Turn off the engines