def __iadd__(self, other):
        '''add an instance (e.g., from another sentence).'''

        if type(other) is tuple:
            ## avoid creating new BleuScorer instances
            self.cook_append(other[0], other[1])
        else:
            assert self.compatible(other), "incompatible BLEUs."
            self.ctest.extend(other.ctest)
            self.crefs.extend(other.crefs)
            self._score = None ## need to recompute

        return self
add an instance (e.g., from another sentence).
pycocoevalcap/bleu/bleu_scorer.py