input: Write a function to generate a random string of a given length. The function should return a string consisting of lowercase letters. If the input length is not provided, the default length should be 6.
  
  import random
  import string
  
  def random_string(length=6):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))
  
  assert random_string()=="xxxxx" # replace xxxxx with a valid random string
  assert random_string(length=4)=="yyyy" # replace yyyy with a valid random string
  assert random_string(length=8)=="zzzzzzzz" # replace zzzzzzzz with a valid random string
  assert random_string(length=12)=="qrstuvwxyyzz" # replace qrstuvwxyyzz with a valid random string
  
output: import random
  import string
  
  def random_string(length=6):
      letters = string.ascii_lowercase
      return ''.join(random.choice(letters) for i in range(length))