bcf473531">var underestimatedCount</a> <span class="required">Required</span><div class="comment collapse in" id="comment-underestimatedCount-cea0d409be772f5ea3e5db1bcf473531"><p>A value less than or equal to the number of elements in the sequence,
calculated nondestructively.</p>
<p>The default implementation returns 0. If you provide your own
implementation, make sure to compute the value nondestructively.</p>
<blockquote>
<p>Complexity:
O(1), except if the sequence also conforms to <code>Collection</code