ode> can be specified in
two different ways, either <code>fam_vec</code> (the method used
internally in <code>estimate_liability</code>) or with
<code>n_fam</code>. For <code>fam_vec</code> a vector of strings from
the list of possible familial relationships must be provided For the
full list, please see documentation for <code>construct_covmat</code>.
Family members will then appear in the covariance matrix in the same
order as they appear in <code>fam_vec</code>. For <code>n_fam</code> a
named vector is p