s of rules:</p>

<ul>
	<li>Agents interact only through their principal port.</li>
	<li>Each variable in a rule occurs exactly twice, once on each side.</li>
	<li>There is at most one rule for each pair of distinct symbols.</li>
</ul>

<p>In an agent definition, the first port is the principal port,  the rest of the ports are listed in the order obtained by moving anticlockwise round the agent. The following definition follows the interaction net at the left side of the <a href='../media/refs/in_add_rule2.p