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If this was considered a bug: 
 
>>> type(ms("a")+ms("b")) 
<type 'str'> 
 
>>> type(ms("a")[:]) 
<type 'str'> 
 
Are these bugs as well? 
 
I belive this is how the implementation was intended to be, even if not 
optimal for subclasses. 
 
I suggest closing this bug as invalid, and writing a PEP about the possible new 
subclass support change (for all classes), if there's enough interest.