using System;
using System.Collections.Generic;
class Program
{
static void Main() {
HashSet<string> hs = new HashSet<string>();
hs.Add("a");
hs.Add("b");
hs.Add("c");
hs.Add("d");
foreach (var element in hs) {
Console.WriteLine(element);
}
}
}
/*
run:
a
b
c
d
*/