code: def add(self, key: str, value: str):
          
          if key in self:
              # Use super to get mutable set
              super().__getitem__(key).add(value)
          else:
              self[key] = {value}
docs: 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