using System;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Linq;

namespace Test;

​

internal class MainApp

{

    public static void Main()

    {

        HashSet<string> QQ = new(StringComparer.OrdinalIgnoreCase);

        QQ.Add("a");

        QQ.Add("b");

        QQ.Add("c");

        var old = QQ.ToHashSet(StringComparer.OrdinalIgnoreCase); // a b c

        QQ.Add("new"); // insert

        QQ.Remove("a"); // removal

        var insertion = QQ.Except(old); // "b c new" - "a b c" = "new"

        var removal = old.Except(QQ); // "a b c" - "b c new" = "a"

        foreach(var h in insertion) Console.WriteLine(h);

        foreach(var h in removal) Console.WriteLine(h);

        Console.WriteLine("Done");

    }

}