input: Write a function to return a new list containing only the common elements from two input lists. If there are no common elements, return an empty list.
  assert common_elements([1, 2, 3, 4, 8], [2, 6, 8, 10])==[2, 8]
  assert common_elements([1, 2, 3], [4, 5, 6])==[]
  assert common_elements([], [1, 2, 3])==[]
  assert common_elements([1, 2, 3], [])==[]
  assert common_elements([1, 1, 1, 2], [1, 2, 2, 3])==[1, 2]
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))