ass="n">matcher</span><span class="o">.</span><span class="nf">register</span><span class="p">((</span><span class="kt">Optional</span><span class="o">&lt;</span><span class="kt">String</span><span class="o">&gt;</span><span class="p">)</span><span class="o">.</span><span class="k">self</span><span class="p">)</span> <span class="p">{</span> <span class="nv">$0</span> <span class="o">==</span> <span class="nv">$1</span> <span class="p">}</span> <span class="c1">// Could use Matcher.default instead, as it is