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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 add_alias(self, alias, index):
 """ Add an alias pointing to the specified index. Args: alias (str):
 The alias that should point to the given index. index (int):
 The index of the dataset for which an alias should be added. Raises: DataInvalidIndex: If the index does not represent a valid dataset. """

        if index >= len(self._datasets):
            raise DataInvalidIndex('A dataset with index {} does not exist'.format(index))
        self._aliases[alias] = index