def cook_append(self, test, refs):
        '''called by constructor and __iadd__ to avoid creating new instances.'''
        
        if refs is not None:
            self.crefs.append(cook_refs(refs))
            if test is not None:
                cooked_test = cook_test(test, self.crefs[-1])
                self.ctest.append(cooked_test) ## N.B.: -1
            else:
                self.ctest.append(None) # lens of crefs and ctest have to match

        self._score = None ## need to recompute
called by constructor and __iadd__ to avoid creating new instances.
torchmm/metrics/caption/bleu/bleu_scorer.py