def remove_common_files(list1, list2):
    """
    returns a subset of list1 that has common elements removed that were found
    in both lists
    or in other words - returns a subset of list1 that has elements unique to
    only list1
    """

    # results in a list of values that are unique to list1
    new_list1 = list(set(list1) - set(list2))
    new_list1.sort()
    return new_list1