     ///
     /// - parameter persistentToken: The persistent token to delete.
     ///
-    /// - returns: A boolean indicating whether the token was successfully deleted.
+    /// - throws: A `Keychain.Error` if the deletion did not succeed.
     public func deletePersistentToken(persistentToken: PersistentToken) throws {
         try deleteKeychainItemForPersistentRef(persistentToken.identifier)
     }

    ///
    /// - parameter token: The persistent identifier for the desired token.
    ///
    /// - returns: The persistent token, or `nil` if no token matched the given identifier.
    public func persistentTokenWithIdentifier(identifier: NSData) throws -> PersistentToken? {
        return try keychainItemForPersistentRef(identifier)

    /// - parameter persistentToken: The persistent token to update.
    /// - parameter token: The new token value.
    ///
    /// - throws: A `Keychain.Error` if the update did not succeed.
    /// - returns: The updated persistent token.
    public func updatePersistentToken(persistentToken: PersistentToken,
        withToken token: Token) throws -> PersistentToken
    {
