Hi, I'm currently implementing the unit systems into sympy (see https://groups.google.com/forum/?fromgroups#!topic/sympy/Vrvwf5CYmWw). The dimensions of an unit are stored in an object which inherits from the dict class. But because of that, when I'm writing something like 2*m, where m is an unit, I got the error "TypeError: unhashable type: dict", appearing in the cache function. I saw by looking at other topics that in principle it happens because the __hash__ method is not defined, but since here the class inherits from dict, there is no reason that it's not defined. I also tried to make the dimension class deriving from something like Basic, Expr, etc., but I have an error saying that there is a clash between methods. I tried to seehow to fix this, but instead of rewriting from the beginning the dimension class.
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