ge &amp;r2)</code></td>
<td>Constructs a sub matrix.</td>
</tr>

<tr>
<td><code>size_type start1 () const</code></td>
<td>Returns the index of the first row.</td>
</tr>

<tr>
<td><code>size_type size1 () const</code></td>
<td>Returns the number of rows.</td>
</tr>

<tr>
<td><code>size_type start2 () const</code></td>
<td>Returns the index of the first column.</td>
</tr>

<tr>
<td><code>size_type size2 () const</code></td>
<td>Returns the number of columns.</td>
</tr>

<tr>
<td><code>const_reference operator