a further example, consider</p>
<pre class="stan"><code>matrix[M, M] x;
vector[M] y;
real z;
x *= x;  // OK, (x * x) is a matrix
x *= z;  // OK, (x * z) is a matrix
x *= y;  // BAD, (x * y) is a vector</code></pre>
<p>The supported compound arithmetic and assignment
operations are listed in the <a href="assignment-statement-section.html#compound-arithmetic-assign-table">compound arithmetic/assignment table</a>; they
are also listed in the index prefaced by <code>operator</code>, e.g.,
<code>operator+=</code