input: Write a python function to concatenate two lists and return the result.
  assert concatenate_lists([1,2,3,4,5], [6,7,8,9,10]) == [1,2,3,4,5,6,7,8,9,10]
  assert concatenate_lists([], [1,2,3]) == [1,2,3]
  assert concatenate_lists([1,2,3], []) == [1,2,3]
  assert concatenate_lists([], []) == []
output: def concatenate_lists(list1, list2):
      return list1 + list2