overload binary + operator to work with different types

[Marc via Digitalmars-d-learn]

I want to basically make this work:

auto l = new List();
l += 5;

I managed to do this:

class List
{
        int[] items;
        ref List opBinary(string op)(int rhs) if(op == "+")
        {
                items ~= rhs;
                return *this;
        }
}

Note the ref in the fucntion return, I also want to return a 
reference to the class so that this Works:

l += 5 + 8 + 9 ... ;

Could someone point out how do that/what's wrong with my attempy?