note35

class Set(object):
    """Sets can contain anything, including themselves. This leads to
    paradoxical behavior: given R, the set of all sets that don't contain
    themselves, does R contain R? Here this becomes an infinite recursion.
    """
    def __init__(self, predicate):
        self.predicate = predicate

    def __contains__(self, obj):
        return self.predicate(obj)