Language: C++

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

using CHY72;

namespace CHY72{
    
    public class PointXY<T>where T:IComparable {
        public T X , Y;
        public bool Flag = false;
        public PointXY(T _X,T _Y) {
            X = _X; Y = _Y;
        }
        public static bool operator ==(PointXY<T> P1, PointXY<T> P2) {
            return (P1.X.CompareTo( P2.X) == 0 && P1.Y.CompareTo (P2.Y) == 0);
        }
        public static bool operator !=(PointXY<T> P1, PointXY<T> P2) {return !(P1 ==P2);}
        public override int GetHashCode() {return X.GetHashCode() ^ Y.GetHashCode();}
        public override bool Equals(object obj) {
            if (obj == null || this.GetType() != obj.GetType()) {return false;}
            return (this == (PointXY<T>)obj);
        }         

    }
    
}    

public class Program {
    static bool ReadLine_1(ref int num1, bool Fin0 = true) {
        string str = Console.ReadLine();
        num1 = int.Parse(str);
        return (Fin0 && num1 == 0);
    }
    static int  ReadLine_1() {
        return  int.Parse(Console.ReadLine());
    }
    static bool ReadLine_2(ref int num1, ref int num2, char Spliter = ' ', bool Fin0 = true) {
        string str = Console.ReadLine();
        num1 = int.Parse(str.Split(Spliter)[0]);
        num2 = int.Parse(str.Split(Spliter)[1]);
        return (Fin0 && num1 == 0);
    }
    static int[] ReadLine72(char Spliter = ' ') {
        return Console.ReadLine().Split(Spliter).Select(s => int.Parse(s)).ToArray();
    }

    const int DefaultMax = 727272;



    static void Main(string[] args) {
        int iMax = ReadLine_1();
        var IDs = new string[iMax];
        //var IDs = new Dictionary<string,bool>();
        for (int i = 0; i < iMax; i++) {
            IDs[i] = Console.ReadLine();
            //IDs.Add( Console.ReadLine(),false );
        }
        iMax = ReadLine_1();
        bool isOpen = false;
        for (int i = 0; i < iMax; i++) {
            string Commer = Console.ReadLine();
            //if (IDs.ContainsKey(Commer)) {
            if (IDs.Contains(Commer)) {
                if (!isOpen ) { Console.WriteLine("Opened by " + Commer); }
                else { Console.WriteLine("Closed by " + Commer); }
                isOpen = !isOpen;
            }else {
                Console.WriteLine("Unknown " + Commer);
            
            }
        
        }

    }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Linq;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Text;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.IO;
      |       ^~~~~~
a.cc:7:7: error: expected nested-name-specifier before 'CHY72'
    7 | using CHY72;
      |       ^~~~~
a.cc:11:5: error: expected unqualified-id before 'public'
   11 |     public class PointXY<T>where T:IComparable {
      |     ^~~~~~
a.cc:31:1: error: expected unqualified-id before 'public'
   31 | public class Program {
      | ^~~~~~