4 8 2018
​y != x - (x-y)
y != x - x + y
y != y

# 非空且不为子父关系的两个集合,符合条件
x = {"a","b","c"}
y = {"b","d"}
{"a","b","c"} - {"a","c"} = {"b"}
{"b","d"} != {"b"}

# 两个集合 , 互为子集父集时,不符合条件
x = {"a","b","c"}
y = {"b","c"}
{"b","c"} != {"a","b","c"} - ( {"a","b","c"} - {"b","c"})----------------------------------------------------------------------------------------------------------------------------------------------------------
{"b","c"} != {"a","b","c"} - {"a"}
{"b","c"} != {"b","c"}

# 两个空集合, 不符合条件
x = set()
y = set()
set() != set() - (set()-set())
set() != set()

​

 

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