he lambda expression 
<code class="literal">i += _1</code> is that calls to the lambda functor affect the 
value of the variable <code class="literal">i</code>, 
rather than some temporary copy of it. 

As another example, the streaming operators take their leftmost argument 
as non-const references. 

The exact rules are:

</p>
<div class="itemizedlist"><ul class="itemizedlist" type="circle">
<li class="listitem"><p>The left argument of compound assignment operators (<code class="literal">+=</code>, <code 