Provide the declaration of subscript operator [] method that allows data modification

The Week class below is essentially a collection of Day objects:

struct Day {
 //.. data members
};

enum Weekday { Sun = 0, Mon, Tue, Wed, Thu, Fri, Sat };

struct Week {

  // overloaded [] operator
  const Day& operator[](Weekday wd) const {
     if(wd < Sun || wd > Sat)
        throw std::out_of_range("Weekday out of range");
     return days[wd];
  }

  // .. more methods

private:
  Day days[7];
};

The above implementation of operator []( ) is good for only reading the Day elements in Week, but it is not possible to modify a Day in the Week through the subscript operator:

Week wk;
//.....
Day d = wk[Sun]; // OK  
wk[Mon] = Day(); // ERROR!!!

We want to change the declaration of operator [](), without changing its implementation, so that the Week's Day elements can be modified. Select the appropriate declaration that would work: