input: Write a python function to check if all elements in the list are equal.
  assert all_equal([1, 1, 1, 1]) == True
  assert all_equal([1, 2, 1, 1]) == False
  assert all_equal(['a', 'a', 'a']) == True
  assert all_equal(['a', 'b', 'a']) == False
output: def all_equal(lst):
      return all(x == lst[0] for x in lst)