 matrix.
If some lines have different lengths, <code>read_matrix</code> complains.
</p>
<p><code>read_matrix(<var>S</var>, <var>M</var>)</code> read the source <var>S</var> into the matrix <var>M</var>,
until <var>M</var> is full or the source is exhausted.
Input data are read into the matrix in row-major order;
the input need not have the same number of rows and columns as <var>M</var>.
</p>
<p>The source <var>S</var> may be a file name or a stream.
</p>
<p>The recognized values of <var>separator_flag</var