class LimitedSet(object):
    def __init__(self, allowed):
        self.contents = {}
        self.allowed = allowed
    def add(self, element):
        if element in self.allowed:
            self.contents[element] = element
            return True
        return False
    def remove(self, element):
        if element in self.contents:
            del self.contents[element]
            return True
        return False
    def contains(self, element):
        return element in self.contents
    def is_allowed(self, element):
        return element in self.allowed

class LetterSet(LimitedSet):
    def __init__(self):
        LOWERCASE = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
        UPPERCASE = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']
        LimitedSet.__init__(self, LOWERCASE + UPPERCASE)

class NumberSet(LimitedSet):
    def __init__(self):
        DIGITS = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '0']
        LimitedSet.__init__(self, DIGITS)