input: Write a python function to generate a random sequence of 20 lowercase letters.
  assert len(generate_sequence()) == 20
  assert generate_sequence().islower()
  assert all('a' <= char <= 'z' for char in generate_sequence())
output: import random
  import string
  
  def generate_sequence():
      return ''.join(random.choice(string.ascii_lowercase) for _ in range(20))