pan><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span>
</pre>
<p class="simpara">
            <code class="computeroutput"><span class="identifier">expression1</span></code> and <code class="computeroutput"><span class="identifier">expression2</span></code> keep track of their left
            and right operands, and the operation on those operands. The expressions
            keep also track of the result type of the associated sub-expression.
            In the 