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The following code shows that the Counter is not deepcopied properly. The 
same code with an user defined class or a dict is copied with the "b" attribute.
import collections
import copy
count = collections.Counter()
count.b = 3
print(count.b) # prints 3
count_copy = copy.deepcopy(count)
print(count_copy.b) # raise AttributeError: 'Counter' object has no attribute 'b'