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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, source, clean=True):
 """Add an alias, optionally 'cleaning' the alias string. Calls the parent `catalog` method `clean_entry_name` - to apply the same name-cleaning as is applied to entry names themselves. Returns ------- alias : str The stored version of the alias (cleaned or not). """

        if clean:
            alias = self.catalog.clean_entry_name(alias)
        self.add_quantity(self._KEYS.ALIAS, alias, source)
        return alias