>lambda</span> () <span class="mv">exp</span>)</code>.</li>
<li><code>force</code> simply calls the procedure. We can optimize it by saving the result and not calling the procedure a second time.</li>
<li>The promise stored in the <code>cdr</code> of the stream is also known as a <em>thunk</em>.</li>
</ul>
<aside><h4>Exercises:</h4> <ul class="flat"><li class="flat__item"><a href="../../exercise/3/5.html#ex3.50">3.50</a>, </li><li class="flat__item"><a href="../../exercise/3/5.html#ex3.51">3.51</a>, </li><l