ic vector of fixed effects. This can also be
        <code><a href="https://rdrr.io/r/stats/family.html" class="external-link">family</a></code> object. In this case initial values for the fixed effects
        will be calculated by using <code><a href="https://rdrr.io/r/stats/glm.html" class="external-link">glm</a></code> to the data ignoring the
        correlations in the repeated measurements. For example, for a negative binomial
        response outcome, we could set <code>betas = poisson()</code>.</p>