input: Write a function named `random_string` that generates a random string of a specified length. The function should use lowercase English letters. If no length is specified, it should generate a string of length 6 by default.
  
  assert random_string()=='abcdef'
  assert random_string(3)=='xyz'
  assert random_string(6).islower()==True
  assert len(random_string(6))==6
  assert all(c in string.ascii_lowercase for c in random_string(6))==True
output: import random
  import string
  
  def random_string(length=6):
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(length))