input: Write a python function to find common elements in two lists.
  assert common_elements([1, 2, 3], [2, 3, 4]) == [2, 3]
  assert common_elements([5, 6, 7], [7, 8, 9]) == [7]
  assert common_elements([10, 20, 30], [40, 50, 60]) == []
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))