#include <iostream>

using namespace std;

class Interface {
public:
  virtual void print() const = 0;
};

class List : public Interface {
  int data = 1;
public:
  void print() const {
    cout << "hello from list" << '\n';
  }

  int get_data() const {
    return data;
  }
};

class Set : public Interface {
  int data = 0;
public:
  void print() const {
    cout << "hello from set" << '\n';
  }

  int get_data() const {
    return data;
  }

  void add(Interface* interf) {
    List* foo = (List*)interf;
    this->data += foo->get_data();
  }
};


int main() {
  Interface* set = new Set;
  Interface* list = new List;
  set->print();
  list->print();
  printf("The set data before: %d\n", ((Set*)set)->get_data());
  ((Set*)set)->add(list);
  printf("The set data after: %d\n", ((Set*)set)->get_data());
}

interpretate(parse(lex(user_expression))) // return result() - то есть на выходе будет строка

lex -> tokens[]