          <span class="sa">f</span><span class="s1">&#39;</span><span class="si">{</span><span class="nb">type</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">feeder</span><span class="p">)</span><span class="si">}</span><span class="s1">, it must be a function returning &quot;x&quot; and &quot;y&quot; values...&#39;</span><span class="p">)</span>
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