input: Write a function to find the common elements in two lists. The function should take two lists as input and return the list of all common elements. If there are no common elements, it should return an empty list.
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3, 4], [5, 6, 7, 8])==[]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([1, 2, 3], [])==[]
  assert find_common_elements([1, 1, 1], [1, 1, 1])==[1, 1, 1]
output: def find_common_elements(list1, list2):
      return [item for item in list1 if item in list2]