def parse_lists_from_api(list_string, separator=';'):
    """
    Parse list of items returned by the OpenCitations API
    :param list_string: list of dois separated by sep
    :type list_string: str
    :param separator: (Default ';') separator to separate elements in string
    :type separator: str
    :return: items in the string as a list
    :rtype: list
    """
    if list_string is None:
        return None
    items = [item.strip() for item in list_string.split(separator)]
    return items