><span class="op">(</span>fac<span class="op">=</span><span class="fu"><a href="https://rdrr.io/r/base/factor.html" class="external-link">factor</a></span><span class="op">(</span><span class="va">letters</span><span class="op">[</span><span class="fl">1</span><span class="op">:</span><span class="va">nfac</span><span class="op">]</span><span class="op">)</span>, t<span class="op">=</span><span class="fl">1</span><span class="op">:</span><span class="va">nt</span><span class="op">)</span></span></span>
<spa