of the
function. It can be specified in a number of different ways:</p><ul><li><p>A name of a variable in the model: <code>obs_additive("effect")</code></p></li>
<li><p>A compartment concentration: <code>obs_additive(~C["central"])</code></p></li>
<li><p>An equation: <code>obs_additive(~base+slp*time)</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 be useful to make