def add(self, key: str, value: str):
        """
        Adds the given value to the set of data stored under the given key.
        :param key: the set's key
        :param value: the value to add in the set associated to the given key
        """
        if key in self:
            # Use super to get mutable set
            super().__getitem__(key).add(value)
        else:
            self[key] = {value}