* Note: this fixes some issues with the previous implementation of metrics for disks:

	      * Time duration tracked was of the initial API call, not the entire operation.

	      * Metrics label tuple would have resulted in many independent histograms stored, one for each disk. (Did not aggregate well).

must contain exactly one single-valued expression, and the left-hand
expression must not be the blank identifier.
</p>

<pre>
a[i] &lt;&lt;= 2
i &amp;^= 1&lt;&lt;n
</pre>

<p>
A tuple assignment assigns the individual elements of a multi-valued
operation to a list of variables.  There are two forms.  In the
first, the right hand operand is a single multi-valued expression