 whenever the long form is legal.
This requires that the operation <code>x op y</code> must itself be well
formed and that the result of the operation be assignable to <code>x</code>.
For the expression <code>x</code> to be assignable, it must be an indexed
variable where the variable is defined in the current block. For
example, the following compound addition and assignment statement will
increment a single element of a vector by two.</p>
<pre class="stan"><code>vector[N] x;
x[3] += 2;</code></pre>
<p>As 