using System;

using System.IO;

using YamlDotNet.RepresentationModel;

public class Program

{

    public static void Main()

    {

        const string initialContent = "---\nversion: 1\n...";

        var sr = new StringReader(initialContent);

        var stream = new YamlStream();

        stream.Load(sr);

        var rootMappingNode = (YamlMappingNode)stream.Documents[0].RootNode;   

        rootMappingNode.Add("shout", "yay!");

        using (var textWriter = Console.Out)

            stream.Save(textWriter, false);

    }

}