input: Write a function to find common elements in two lists.
  assert common_elements([1, 2, 3], [2, 4, 5]) == [2]
  assert common_elements([10, 20, 30], [5, 10, 15, 20]) == [10, 20]
  assert common_elements(['a', 'b', 'c'], ['d', 'b', 'a']) == ['a', 'b']
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))