 attribute variable, <em class="parameter"><code>index</code></em> refers to the
    first column of the matrix. Other matrix columns are then
    automatically bound to locations <em class="parameter"><code>index+1</code></em>
    for a matrix of type <code class="function">mat2</code>; <em class="parameter"><code>index+1</code></em> and
    <em class="parameter"><code>index+2</code></em> for a matrix of type <code class="function">mat3</code>; and
    <em class="parameter"><code>index+1</code></em>, <em c