using System;
using System.Collections.Generic;

namespace Clones
{
    public class CloneVersionSystem : ICloneVersionSystem
    {
        public class Clone
        {
            public Stack<int> PassedProgramms;
            public Stack<int> Recoil;
        }

        public string Execute(string query)
        {
            var clones = new List<Clone>();
            var command = query.Split(' ');
            var ci = int.Parse(command[1]);
            if (command[0] == "learn")
            {
                clones[ci].PassedProgramms.Push(int.Parse(command[2]));
                clones[ci].Recoil.Clear();
            }
            else if (command[0] == "rollback")
                clones[ci].Recoil.Push(clones[ci].PassedProgramms.Pop());
            else if (command[0] == "relearn")
                clones[ci].PassedProgramms.Push(clones[ci].Recoil.Pop());
            else if (command[0] == "clone")
                clones.Add(new Clone { PassedProgramms = clones[ci].PassedProgramms, Recoil = clones[ci].Recoil });
            else if (command[0] == "check")
            {
                if (clones[ci].PassedProgramms.Count == 0)
                    return "basic";
                var lastProgramm = clones[ci].PassedProgramms.Pop();
                clones[ci].PassedProgramms.Push(lastProgramm);
                return lastProgramm.ToString();
            }
            return null;
        }
    }
}