> notation within model formulae, can lead to confusing (and sometimes hard-to-detect) errors.</p></li>
<li><p>By default, vector-valued random effects are fitted with unstructured (general symmetric positive definite) variance-covariance matrices. Structured variance-covariance matrices can be specified in the form <code>struc(terms|group)</code>, where <code>struc</code> is one of</p><ul><li><p><code>diag</code> (diagonal, heterogeneous variance)</p></li>
<li><p><code>ar1</code> (autoregressive order-1, h