var document = new TextDocument();
document.Language = CSharpLanguage.Instance;
// ... Parse a document
var originalTree = document.SyntaxTree;
var node1 = originalTree.RootNode.GetChild(0);
var node2 = originalTree.RootNode.GetChild(1);
var annotatedTree = originalTree.AddAnnotation(node1, new SyntaxAnnotation());
var annotatedTreeAgain = annotatedTree.AddAnnotation(node2, new SyntaxAnnotation());