>A name of a variable in the model: <code><a href='obs_additive.html'>obs_additive("effect")</a></code></p></li>
<li><p>A compartment concentration: <code><a href='obs_additive.html'>obs_additive(~C["central"])</a></code></p></li>
<li><p>An equation: <code><a href='obs_additive.html'>obs_additive(~base+slp*time)</a></code></p></li>
</ul>

<p>If the definition contains a variable name on the left-hand side (as in
<code>conc~C["central"]</code>), the variable will appear in the generated model
code. This can 