public async Task ParameterTest_ComputeCurrentParameter01()
        {
            var textBuffer   = new TextBufferMock("aov(", RContentTypeDefinition.ContentType);
            var editorBuffer = textBuffer.ToEditorBuffer();
            var source       = new RSignatureHelpSource(textBuffer, Services);
            var session      = new SignatureHelpSessionMock(Services, textBuffer, 0);
            var textView     = session.TextView as TextViewMock;
            var signatures   = new List <ISignature>();

            using (var tree = new EditorTree(editorBuffer, Services)) {
                tree.Build();
                using (var document = new EditorDocumentMock(tree)) {
                    session.TrackingPoint = new TrackingPointMock(textBuffer, 4, PointTrackingMode.Positive, TrackingFidelityMode.Forward);
                    await FunctionIndex.GetPackageNameAsync("aov");

                    tree.TakeThreadOwnerShip();
                    await source.AugmentSignatureHelpSessionAsync(session, signatures, tree.AstRoot);

                    signatures.Should().ContainSingle();

                    var index = GetCurrentParameterIndex(signatures[0], signatures[0].CurrentParameter);
                    index.Should().Be(0);

                    textView.Caret = new TextCaretMock(textView, 5);
                    TextBufferUtility.ApplyTextChange(textBuffer, 4, 0, 1, "a");
                    index = GetCurrentParameterIndex(signatures[0], signatures[0].CurrentParameter);
                    index.Should().Be(0);

                    textView.Caret = new TextCaretMock(textView, 6);
                    TextBufferUtility.ApplyTextChange(textBuffer, 5, 0, 1, ",");
                    tree.EnsureTreeReady();
                    index = GetCurrentParameterIndex(signatures[0], signatures[0].CurrentParameter);
                    index.Should().Be(1);

                    textView.Caret = new TextCaretMock(textView, 7);
                    TextBufferUtility.ApplyTextChange(textBuffer, 6, 0, 1, ",");
                    tree.EnsureTreeReady();
                    index = GetCurrentParameterIndex(signatures[0], signatures[0].CurrentParameter);
                    index.Should().Be(2);
                }
            }
        }