S(Z))), or 3</span>
</pre>

<p>When defining multiplication, note that the argument <i>y</i> is used twice in the first equation, and it is not used at all in the second one. For that reason, two extra symbols are needed <i>duplicate</i> and <i>erase</i>. </p>

<pre>
sx * y = (x * y) + y               0 * y = 0
</pre>
<img src='../media/refs/in_mul.png' width='450'/>

<p>The idea is that a net representing a natural number should be duplicated when it is connected to the principal port of a <i>duplicate</i>