//This method has potential for some micro-optimizations. Somebody *please*
    //convince me *not* to do them.
    void arg(const string& a) {
        if (next_arg >= 9) {
            return;
        }
        size_t offset = offsets[next_arg];

        str.erase(offset, 2); //erase the placeholder
        str.insert(offset, a); //insert the new string in place

        next_arg++;
    }