input: def sequence_string(n: int) -> str:
      """ Produce a string of space-separated integers starting from 0 up to and including n.
      >>> sequence_string(0)
      '0'
      >>> sequence_string(5)
      '0 1 2 3 4 5'
      """
output:     return ' '.join(str(i) for i in range(n + 1))