input: Write a function to generate a random string of a given length. The string should consist of lowercase alphabetic characters. If the given length is negative, return an empty string.
  
  def random_string(length):
      if length < 0:
          return ""
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert random_string(6) != random_string(6)
  assert random_string(0) == ""
  assert random_string(-1) == ""
output: def random_string(length):
      if length < 0:
          return ""
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))