int are_they_friends(struct user a, struct user b) {
    // check if b is in a's friends list
    int a_friends_num = sizeof(a.friends) / sizeof(a.friends[0]);
    for (int i = 0; i < a_friends_num; i++) {
        if (a.friends[i] == b) {
            return 0; //true
        }
    }
    return 1; //false
}
checks whether user b is in an array attribute, friends, of user a