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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_filename(self, variable):
 """Return the auxiliary file name the given variable is allocated to or |None| if the given variable is not allocated to any auxiliary file name. 'file1' """

        fn2var = self._type2filename2variable.get(type(variable), {})
        for (fn_, var) in fn2var.items():
            if var == variable:
                return fn_
        return None