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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(self, value):
 # type: (Text) -> Text """ Add the given value to the store. Returns a placeholder value to use until the real value is needed. """

        if not isinstance(value, string_types):
            raise Exception("Secret store only accepts strings")

        if value not in self.secrets:
            placeholder = "(secret-%s)" % Text(uuid.uuid4())
            self.secrets[placeholder] = value
            return placeholder
        return value