nd column extraction for matrices</h3>
<p>Matrix slicing can also be performed using the <code>block</code> function. For example,</p>
<pre><code>matrix[20, 20] a = ...;
matrix[3, 2] b = block(a, 5, 9, 3, 2);</code></pre>
<p>will set <code>b</code> equal to the submatrix of <code>a</code> starting at index [5, 9] and extending 3 rows and 2 columns. Thus <code>block(a, 5, 9, 3, 2)</code> is equivalent to <code>b[5:7, 9:10]</code>.</p>
<p>The <code>sub_col</code> function extracts a slice of a column of a mat