input: Write a python function to flatten a list of lists into a single list.
  assert flatten([[1, 2, 3], [4, 5], [6, 7, 8, 9]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert flatten([[], [1, 2], [3, 4, 5]]) == [1, 2, 3, 4, 5]
  assert flatten([[1], [2, 3], [], [4, 5, 6]]) == [1, 2, 3, 4, 5, 6]
output: def flatten(lst):
      return [item for sublist in lst for item in sublist]