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I'm with Raymond on this: the tutorial is intended for 
beginners, and I'm very happy to have beginners believe that 
only immutable objects can be keys. IMHO (which could easily 
be wrong in this case), more advanced users will realize that 
the existance of __hash__ makes it POSSIBLE to make 
mutables hashable, although they may well need additional 
documentation to figure out when doing so will work and 
when it will break things. But this level of detail doesn't 
belong in the tutorial (perhaps in a FAQ someplace?).