using System;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
	static IEnumerable<T> Join<T>(T separator, IEnumerable<T> items) {
		bool first = true;
		foreach (var item in items) {
			if (!first) {
				yield return separator;
			} else {
				first = false;
			}
			yield return item;
		}
	}
	public static void Main()
	{
		List<int> list = new List<int> {1, 2, 3, 4, 5};
		foreach (var x in Join(-1, list)) {
			Console.WriteLine(x);
		}
	}
}

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