using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;

public class Test {
	public static void Main() {
            var A = new VisibleIterator("A");
            var B = new VisibleIterator("B");
            var C = new VisibleIterator("C");
            var D = new VisibleIterator("D");
            var foo = (A.Union(B).Union(C).Union(D)).ToList();
            Console.WriteLine("---");
            var bar = (A.Union(B.Union(C.Union(D)))).ToList();
            Console.WriteLine("---");
            var baz = (D.Union(C.Union(B.Union(A)))).ToList();
	}
}

    class VisibleIterator : IEnumerable<string> {
        private readonly string name;
        public VisibleIterator(string name) {
            this.name = name;
        }
        public IEnumerator<string> GetEnumerator() {
            for (var i = 0 ; i != 4 ; i++) {
                var res = name+i;
                Console.WriteLine(res);
                yield return res;
            }
        }
        IEnumerator IEnumerable.GetEnumerator() {
            return GetEnumerator();
        }
    }