private static async Task <Document> AddPartialModifier(Document document, TypeDeclarationSyntax typeDecl, CancellationToken cancellationToken)
{
var newTypeDecl = typeDecl.AddPartialModifier();
var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false) as CompilationUnitSyntax;
var newRoolt = root.ReplaceNode(typeDecl, newTypeDecl)
.WithAdditionalAnnotations(Formatter.Annotation);
document = document.WithSyntaxRoot(newRoolt);
return(document);
}