     }
 
     /// Returns an array of all persistent tokens found in the keychain.
+    ///
+    /// - throws: A `Keychain.Error` if an error occurred.
     public func allPersistentTokens() throws -> [PersistentToken] {
         guard let keychainItems = try allKeychainItems() as? [NSDictionary] else {
             throw Error.IncorrectReturnType

    ///
    /// - parameter persistentToken: The persistent token to delete.
    ///
    /// - returns: A boolean indicating whether the token was successfully deleted.
    public func deletePersistentToken(persistentToken: PersistentToken) throws {
        try deleteKeychainItemForPersistentRef(persistentToken.identifier)
    }

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