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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 _checkAndConvertIndex(self, index):
 """Check integer index, convert from less than zero notation """

        if index < 0:
            index = len(self) + index
        if index < 0 or index >= self._doc.blockCount():
            raise IndexError('Invalid block index', index)
        return index