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