">pair_counts</span><span class="p">[</span><span class="n">type2</span><span class="p">,</span> <span class="n">type1</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span>
            <span class="k">else</span><span class="p">:</span>
                <span class="n">pair_counts</span><span class="p">[</span><span class="n">type1</span><span class="p">,</span> <span class="n">type2</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span>
              