private Task<Document> AddStaticKeyword(Document document, SyntaxNode root, ClassDeclarationSyntax classDeclaration)
 {
     var staticKeyword = SyntaxFactory.Token(SyntaxKind.StaticKeyword).WithAdditionalAnnotations(Formatter.Annotation);
     var newDeclaration = classDeclaration.AddModifiers(staticKeyword);
     var newRoot = root.ReplaceNode(classDeclaration, newDeclaration);
     return Task.FromResult(document.WithSyntaxRoot(newRoot));
 }
        private async Task<Document> AddSealedAsync(Document document,
                                                    ClassDeclarationSyntax classDeclaration,
                                                    CancellationToken cancellationToken)
        {
            var sealedKeyword = SyntaxFactory.Token(SyntaxKind.SealedKeyword).WithAdditionalAnnotations(Formatter.Annotation);
            var newClass = classDeclaration.AddModifiers(sealedKeyword);

            var root = await document.GetSyntaxRootAsync(cancellationToken);
            var newRoot = root.ReplaceNode(classDeclaration, newClass);
            var newDocument = document.WithSyntaxRoot(newRoot);

            return newDocument;
        }