When x Is not an empty set and y No x Is true when a subset of .
x={
'd','b'}
y={
'a','c'}
print(y!=x-(x-y))
Output ：
True
y yes x A subset of ：
x={
'a','c','d'}
y={
'a','c'}
print(y!=x-(x-y))
Output ：
False
Non empty sets are easy to understand , The result of two empty sets calculation is still an empty set , So it will be True.
Note that floating-point errors do not cause inequality , Because the error is consistent , Some words will be subtracted at the same time .
