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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 setdefault(self, key, default=None):
 """Return value for key, set value to default if key is not present."""

        identity = self._title(key)
        for i, k, v in self._impl._items:
            if i == identity:
                return v
        self.add(key, default)
        return default