input: Write a function to take a positive integer and output the numbers counting down from that integer to 1 using a while loop. If the input is not a positive integer, print an error message.
  assert count_down(6)
  assert count_down(3)
  assert count_down(-1)
  assert count_down(0)
  def count_down(n):
      if n <= 0:
          print("Error: Input must be a positive integer.")
      else:
          while n > 0:
              print(n)
              n -= 1
          print("Done!")
      return
output: def count_down(n):
      if n <= 0:
          print("Error: Input must be a positive integer.")
      else:
          while n > 0:
              print(n)
              n -= 1
          print("Done!")