Add the combine and isbn members to the Sales_data class you wrote for the exercises in § 2.6.2 (p. 76)

#include <string>
#include <iostream>

struct Sales_data {
  std::string isbn() const { return bookNo; }
  Sales_data &combine(const Sales_data &);

  std::string bookNo;
  unsigned units_sold = 0;
  double revenue = 0.0;
};

Sales_data &Sales_data::combine(const Sales_data &rhs) {
  units_sold += rhs.units_sold;
  revenue += rhs.revenue;
  return *this;
}


int main() {
  return 0;
}