public async Task ExternalCancellation_MutationOperationSent_WaitsForInternalCancellation()
        {
            // Arrange

            var operation = new Set <dynamic>("fake", "fakeKey")
            {
                IsSent = true
            };

            var externalCts = new CancellationTokenSource(100);
            var internalCts = new CancellationTokenSource();
            var tokenPair   = new CancellationTokenPair(externalCts.Token, internalCts.Token);

            // Act

            using var registration = new OperationCancellationRegistration(operation, tokenPair);
            tokenPair.Register(() => internalCts.CancelAfter(100));

            // Assert

            await Assert.ThrowsAnyAsync <OperationCanceledException>(() => operation.Completed.AsTask());

            Assert.True(internalCts.IsCancellationRequested);
        }