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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 coderef_to_ecoclass(self, code, reference=None):
 """ Map a GAF code to an ECO class Arguments --------- code : str GAF evidence code, e.g. ISS, IDA reference: str CURIE for a reference for the evidence instance. E.g. GO_REF:0000001. Optional - If provided can give a mapping to a more specific ECO class Return ------ str ECO class CURIE/ID """

        mcls = None
        for (this_code,this_ref,cls) in self.mappings():
            if str(this_code) == str(code):
                if this_ref  == reference:
                    return cls
                if this_ref is None:
                    mcls = cls
                    
        return mcls