def custom_list(str_list):
    """
    Converts a set of comma separated values into a set.
    If only one element is found and the element is 'all' then it returns and empty set
    Args:
        str_list:   string representing the set

    Returns:    a set that is the result of the conversion

    """
    values_set = set([x.lower().strip() for x in str_list.split(',') if x])
    if len(values_set) == 1 and 'all' in values_set:
        values_set.clear()
    return values_set