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

namespace ConsoleApp1
{
    static class Class1
    {
        private static IEnumerable<TNode> GetChildren<TNode>(
            TNode node,
            Func<TNode, IEnumerable<TNode>> GetNodes)
        {
            var nodes = GetNodes(node);
            return nodes.Concat(nodes.SelectMany(x => GetChildren(x, GetNodes)));
        }

        public static IEnumerable<TNode> GetChildrenRecursively<TRoot, TNode>(
            this TRoot obj,
            Func<TRoot, IEnumerable<TNode>> EnumRoot,
            Func<TNode, IEnumerable<TNode>> GetNodes)
        {
            var nodes = EnumRoot(obj);
            return nodes.Concat(nodes.SelectMany(x => GetChildren(x, GetNodes)));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            int[] arr = { 1, 2, 3, 4 };
            var query = arr.GetChildrenRecursively<IEnumerable<int>, IEnumerable<int>>(x => arr.Select(y => new int[] { y }), x => arr.Except(x).Select(y => x.Concat(new int[] { y }.ToArray())));
            foreach (var item in query)
            {
                foreach (var i in item) Console.Write(i + " ");
                Console.WriteLine();
            }
        }
    }
}