f the QR-decoposition.
The functions <code>qr_Q</code> and <code>qr_R</code> implement the fat QR decomposition so
here we thin it by including only <span class="math inline">\(K\)</span> columns in <span class="math inline">\(Q\)</span> and <span class="math inline">\(K\)</span> rows in
<span class="math inline">\(R\)</span> (see the reference manual for more information on the <code>qr_Q</code> and
<code>qr_R</code> functions). Also, in practice, it is best to write <span class="math inline">\(x = Q^\ast 