>logical or a list. If logical and equal to <code>FALSE</code>, then no penalty
    is used. If logical and equal to <code>TRUE</code>, for the fixed effects a Student's-t
    penalty/prior with mean 0, scale equal to 1 and 3 degrees of freedom is used. If a
    list, then it is expected to have the components <code>pen_mu</code>, <code>pen_sigma</code> and
    <code>pen_df</code>, denoting the mean, scale and degrees of freedom of the Student's-t
    penalty/prior for the fixed effects.</p></dd>

<dt id="a