ative of <code>fct</code> with respect to <code>par</code>.
It also uses the variance-covariance matrix of the estimated parameters
which is derived in estimating the parameters and is an input argument.</p>
<p>The first argument of <code>fct</code> should be <code>par</code> which is a vector of
parameter estimates. It should return a single value (or vector) of
estimate(s).  The remaining arguments of <code>fct</code> if any can be passed to
<code>fct</code> by including them at the end of the call to <co