def is_subset_of(main):
    """
    Validate that the given set is subset of the main given set.

    >>> f = is_subset_of(set(['a', 'b', 'c', 'd']))
    >>> f(set(['b', 'd']))
    True
    >>> f(set(['a', 'b', 'c', 'd']))
    True
    >>> f(set(['a', 'f']))
    False

    :param set main: The main set to compare to.
    :rtype: A validator function.
    """
    def validator(sub):
        return sub <= main
    return validator