private async Task<Document> MarkClassAsStatic(Document document, ClassDeclarationSyntax classDeclaration, CancellationToken c)
    {
        SyntaxToken classDeclarationToken = classDeclaration.Keyword;
        SyntaxToken staticKeywordToken = SyntaxFactory.Token(SyntaxKind.StaticKeyword);

        SyntaxNode root = await document.GetSyntaxRootAsync();
        SyntaxNode newRoot = root.InsertTokensBefore(classDeclarationToken, new[] { staticKeywordToken });

        Document newDocument = document.WithSyntaxRoot(newRoot);
        return newDocument;
    }

        ClassDeclarationSyntax newClassDeclaration = classDeclaration.AddModifiers(new SyntaxToken[] { staticKeywordToken });

        SyntaxNode root = await document.GetSyntaxRootAsync();
        SyntaxNode newRoot = root.ReplaceNode(classDeclaration, newClassDeclaration);