  Seymour's Second Neighborhood Conjecture asserts that every oriented graph
has a vertex whose first out-neighborhood is at most as large as its second
out-neighborhood. Combs are the graphs having no induced $C_4$,
$\overline{C_4}$, $C_5$, chair or $\overline{chair}$. We characterize combs
using dependency digraphs. We characterize the graphs having no induced $C_4$,
$\overline{C_4}$, chair or $\overline{chair}$ using dependency digraphs. Then
we prove that every oriented graph missing a comb satisfies this conjecture. We
then deduce that every oriented comb and every oriented threshold graph
satisfies Seymour's conjecture.
