Combination of ElementAt and Add method
<pre><code>int k = 0;

List&lt;HashSet&lt;string&gt;&gt; bela = new List&lt;HashSet&lt;string&gt;&gt;();
line = Regex.Split(" file contents", @"\s+");

for (int j = 0; j &lt; line.Length; j++)
{
   bela.ElementAt(k++).Add(lineTokens[j]);
}
</code></pre>

<p>Can you explain the meaning of this line ?
bela.ElementAt(k++).Add(lineTokens[j]);</p>
