英文文档：
class frozenset([iterable])
Return a new frozenset object, optionally with elements taken from iterable. frozenset is a built-in class. See frozenset and Set Types — set, frozenset for documentation about this class.
For other containers see the built-in set, list, tuple, and dict classes, as well as the collections module.
说明：
1. 传入一个可迭代对象，生成一个新的不可变集合。
>>> a = frozenset(range(10))
>>> a
frozenset({0, 1, 2, 3, 4, 5, 6, 7, 8, 9})
>>> b = frozenset('I am a Pythoner')
>>> b
frozenset({'y', 'I', ' ', 'r', 't', 'm', 'h', 'o', 'a', 'e', 'n', 'P'})2. 不传入参数时，生成的空的不可变集合。
>>> c = frozenset() >>> c frozenset()