, and it should be erased when it is connected to the principal port of an <i>erase</i>.</p>

<img src='../media/refs/in_dupera.png' width='450'/>

<p>The system of interaction combinators consists of three symbols, called combinators: <i>y</i>(constructor), <i>d</i>(duplicator), and <i>e</i>(eraser). The six interaction rules below are of two kinds: commutation when the two cells carry different symbols (<i>yd</i>, <i>ye</i>, <i>de</i>) and annihilation when they carry the same symbol (<i>yy</i>, <i>dd</i>