  We compute the class of the closure of the locus of hyperelliptic curves in
the moduli space of stable genus-3 curves in terms of the tautological class
$\lambda$ and the boundary classes $\delta_0$ and $\delta_1$. The expression of
this class is known, but here we compute it directly, by means of Porteous
Formula, without resorting to blowups or test curves.
