tching two residues, store 1 to 1 mapping.</span>
        <span class="c1"># Store positions as strings, since pandas cannot</span>
        <span class="c1"># handle nan values in integer columns</span>
        <span class="k">if</span> <span class="n">res_i</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">gaps</span> <span class="ow">and</span> <span class="n">res_j</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">gaps</span><span class="p">:</span>