perator+</code>, but it
    is not an efficient one. An unnamed local copy of <code>lhs</code> is
    created, <code>operator+=</code> is called on it and it is copied to the
    function return value (which is another unnamed object of type
    <code>T</code>). The standard doesn't generally allow the intermediate
    object to be optimized away:

    <blockquote>
      3.7.2/2: Automatic storage duration<br>
      <br>
       If a named automatic object has initialization or a destructor with
      side e