def add(self, name, mech, usage='both',
            init_lifetime=None, accept_lifetime=None, impersonator=None,
            store=None):
        """Acquire more credentials to add to the current set

        This method works like :meth:`acquire`, except that it adds the
        acquired credentials for a single mechanism to a copy of the current
        set, instead of creating a new set for multiple mechanisms.
        Unlike :meth:`acquire`, you cannot pass None desired name or
        mechanism.

        If the `impersonator` argument is used, the credentials will
        impersonate the given name using the impersonator credentials
        (:requires-ext:`s4u`).

        If the `store` argument is used, the credentials will be acquired
        from the given credential store (:requires-ext:`cred_store`).
        Otherwise, the credentials are acquired from the default store.

        The credential store information is a dictionary containing
        mechanisms-specific keys and values pointing to a credential store
        or stores.

        Note that the `store` argument is not compatible with the
        `impersonator` argument.

        Args:
            name (Name): the name associated with the
                credentials
            mech (OID): the desired :class:`MechType` to be used with the
                credentials
            usage (str): the usage for the credentials -- either 'both',
                'initiate', or 'accept'
            init_lifetime (int): the desired initiate lifetime of the
                credentials, or None for indefinite
            accept_lifetime (int): the desired accept lifetime of the
                credentials, or None for indefinite
            impersonator (Credentials): the credentials to use to impersonate
                the given name, or None to not acquire normally
                (:requires-ext:`s4u`)
            store (dict): the credential store information pointing to the
                credential store from which to acquire the credentials,
                or None for the default store (:requires-ext:`cred_store`)

        Returns:
            Credentials: the credentials set containing the current credentials
                and the newly acquired ones.

        Raises:
            BadMechanismError
            BadNameTypeError
            BadNameError
            DuplicateCredentialsElementError
            ExpiredCredentialsError
            MissingCredentialsError
        """

        if store is not None and impersonator is not None:
            raise ValueError('You cannot use both the `impersonator` and '
                             '`store` arguments at the same time')

        if store is not None:
            if rcred_cred_store is None:
                raise NotImplementedError("Your GSSAPI implementation does "
                                          "not have support for manipulating "
                                          "credential stores")
            store = _encode_dict(store)

            res = rcred_cred_store.add_cred_from(store, self, name, mech,
                                                 usage, init_lifetime,
                                                 accept_lifetime)
        elif impersonator is not None:
            if rcred_s4u is None:
                raise NotImplementedError("Your GSSAPI implementation does "
                                          "not have support for S4U")
            res = rcred_s4u.add_cred_impersonate_name(self, impersonator,
                                                      name, mech, usage,
                                                      init_lifetime,
                                                      accept_lifetime)
        else:
            res = rcreds.add_cred(self, name, mech, usage, init_lifetime,
                                  accept_lifetime)

        return Credentials(res.creds)
Acquire more credentials to add to the current set

        This method works like :meth:`acquire`, except that it adds the
        acquired credentials for a single mechanism to a copy of the current
        set, instead of creating a new set for multiple mechanisms.
        Unlike :meth:`acquire`, you cannot pass None desired name or
        mechanism.

        If the `impersonator` argument is used, the credentials will
        impersonate the given name using the impersonator credentials
        (:requires-ext:`s4u`).

        If the `store` argument is used, the credentials will be acquired
        from the given credential store (:requires-ext:`cred_store`).
        Otherwise, the credentials are acquired from the default store.

        The credential store information is a dictionary containing
        mechanisms-specific keys and values pointing to a credential store
        or stores.

        Note that the `store` argument is not compatible with the
        `impersonator` argument.

        Args:
            name (Name): the name associated with the
                credentials
            mech (OID): the desired :class:`MechType` to be used with the
                credentials
            usage (str): the usage for the credentials -- either 'both',
                'initiate', or 'accept'
            init_lifetime (int): the desired initiate lifetime of the
                credentials, or None for indefinite
            accept_lifetime (int): the desired accept lifetime of the
                credentials, or None for indefinite
            impersonator (Credentials): the credentials to use to impersonate
                the given name, or None to not acquire normally
                (:requires-ext:`s4u`)
            store (dict): the credential store information pointing to the
                credential store from which to acquire the credentials,
                or None for the default store (:requires-ext:`cred_store`)

        Returns:
            Credentials: the credentials set containing the current credentials
                and the newly acquired ones.

        Raises:
            BadMechanismError
            BadNameTypeError
            BadNameError
            DuplicateCredentialsElementError
            ExpiredCredentialsError
            MissingCredentialsError