def to_list(possible_lst):
    """
    Coerce argument to a list by all means.

    Lists are returned unchanged, strings return a list with a single element,
    and otherwise an empty list is returned.
    """
    if isinstance(possible_lst, list):
        return possible_lst
    elif isinstance(possible_lst, str):
        return [possible_lst]
    else:
        return []
