C++のクラスについての質問です。

①friend bool operator <=, >=, ==のところなのですが.
　①(<=).もしs1の中にs2の数字が全てあるならReturn True.
　②(>=).もしs2のなかにs1の数字が全てあるなら Return true.
　③(==)もしs1とs2の中の数字が一緒なら　Return true にしたいのですがうまくいきません。

②void remove アレイから特定の数字を抜く（この場合はs4の中から99を抜く）と
void clear アレイの数字を全て消す（s1の中の全ての数字）
というファンクションを作りたいのですが全くうまくいきません。（そもそもvoidで正しいのでしょうか…？）

エラーが出るのは②だけです。分かる方がいたら教えてもらえると助かります。よろしくお願いします。

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <fstream>
#include <string>
using namespace std;

class MySet
{
public:
    MySet();
    friend bool operator ^ (int element, const MySet &s);
    friend MySet operator + (MySet s, int element);       
    friend MySet operator + (const MySet &s1, const MySet &s2);
    friend ostream& operator << (ostream &out, const MySet &s);
    friend MySet operator * (const MySet &s1, const MySet &s2);
    friend MySet operator - (const MySet &s1, const MySet &s2);
    friend bool operator <= (const MySet &s1, const MySet &s2); // subset
    friend bool operator >= (const MySet &s1, const MySet &s2); //supterset
    friend bool operator == (const MySet &s1, const MySet &s4);
    void remove(int element, const MySet &s);
    void clear(const MySet &s);
    int size() const;
private:
    int numbers[100];
    int count; //have to be size?
};

int main()
{
      MySet s1;
      cout << "s1: " << s1 << endl; //should print {}
      if (s1.size() == 0) {
            cout << "Set 1 is empty" << endl;
      }
      s1 = s1 + 10; //add an element to s1
      s1 = s1 + 10;
      s1 = s1 + 20;
      cout << "S1: " << s1 << endl;//should print {10, 20}
      if (10 ^ s1) //test membership
      {
            cout << "10 is an element of S1: " << s1 << endl;
      }  
      MySet s2;
      //add elements to s2
      s2 = s2 + 10;
      s2 = s2 + 40;
      s2 = s2 + 50;
      s2 = s2 + 20;
      cout << "S2: " << s2 << endl;
      //S3 is the union of s1 and s2
      MySet s3 = s1 + s2;
      cout << "S3 (S1 + S2): " << s3 << endl;
      //s3 is the intersection of s1 and s2
      s3 = s1 * s2;
      cout << "S3 (S1 * S2): " << s3 << endl;
      //s3 is the differenct of s1 and s2
      s3 = s1 - s2;
      cout << "S3 (S1 - S2): " << s3 << endl;
      //test the subset operation
      if (s1 <= s2)
      {
            cout << "S1 is a subset of S2" << endl;
      }
      //test the superset operation
      if (s2 >= s1)
      {
            cout << "S2 is a superset of s1" << endl;
      }
      MySet s4;
      s4 = s4 + 20;
      s4 = s4 + 10;
      cout << "S4: " << s4 << endl;
      //test equality
      if (s1 == s4)
      {
            cout << "S1 equals S4" << endl;
      }
      s4 = s4 + 99;
      s4 = s4 + 100;
      cout << "S4: " << s4 << endl;
      s4.remove(99);
      cout << "S4 after removing 99: " << s4 << endl;
      s1.clear();
      cout << "After clearing s1: " << s1 << endl; //should print {}
      return 0; //or EXIT_SUCCESS
}

MySet::MySet()
{
    count = 0 ;
}

bool operator ^ (int element, const MySet &s)
{
    for(int i = 0; i < s.count; ++i)
    {
        if(element == s.numbers[i])
        {
            return true;
        }
    }
    return false;
}

MySet operator + (MySet s, int element)
{
    if(!(element ^ s))    //if one is T and other is F = T, both T or F = F 
    {
        s.numbers[s.count] = element;
        s.count++;
    }
    return s;
}

MySet operator + (const MySet &s1, const MySet &s2)
{
    MySet s3;
    for(int i = 0; i < s1.count; ++i)
    {
        s3 = s3 + s1.numbers[i];
    }
    for(int i = 0; i < s2.count; ++i)
    {
        s3 = s3 + s2.numbers[i];
    }
    return s3;
}

ostream& operator << (ostream &out, const MySet &s)
{
    out << "{";
    for(int i = 0; i < s.count; ++i)
    {
        out << s.numbers[i];
        if(!(i == (s.count - 1)))
        {
            out << ", ";
        }
    }
    out << "}";
    return out;
}

MySet operator * (const MySet &s1, const MySet &s2)
{
    MySet s3;
    for(int i = 0; i < s1.count; ++i)
    {
        if(s1.numbers[i] ^ s2)
        {
            s3 = s3 + s1.numbers[i];
        }
    }
    return s3;
}

MySet operator - (const MySet &s1, const MySet &s2)
{
    MySet s3;
    for(int i = 0; i < s1.count; ++i)
    {
        if(!(s1.numbers[i] ^ s2))
        {
            s3 = s3 + s1.numbers[i];
        }
    }
    return s3;
}

bool operator <= (const MySet &s1, const MySet &s2)  // subset
{
    for(int i = 0; i < s1.count; ++i)
    {
        if(!(s1.numbers[i] ^ s2))
        {
            return false;
        }
    }
    return true;
}

bool operator >= (const MySet &s1, const MySet &s2) //supterset
{
    for(int i = 0; i < s2.count; ++i)
    {
        if(!(s2.numbers[i] ^ s1))
        {
            return false;
        }
    }
    return true;
}

bool operator == (const MySet &s1, const MySet &s4)
{
    for(int i = 0; i < s1.count; ++i)
    {
        if(!(s1.numbers[i] ^ s4))
        {
            return false;
        }
    }
    for(int i = 0; i < s4.count; ++i)
    {
        if(!(s4.numbers[i] ^ s1))
        {
            return false;
        }
    }
    return true;
}

int MySet::size() const
{
    return count;
}

void MySet::remove(int element, const MySet &s)
{
        for(int i = 0; i < s.count; ++i)
        {
            if(s.numbers[i] == element)
            {
                s.numbers[i] = NULL;
            }
        }
}

void MySet::clear(const MySet &s)
{
    s.clear();
}