  Given a graph $G=(V,E)$ and a weight function on the edges $w:E\mapsto\RR$,
we consider the polyhedron $P(G,w)$ of negative-weight flows on $G$, and get a
complete characterization of the vertices and extreme directions of $P(G,w)$.
As a corollary, we show that, unless $P=NP$, there is no output polynomial-time
algorithm to generate all the vertices of a 0/1-polyhedron. This strengthens
the NP-hardness result of Khachiyan et al. (2006) for non 0/1-polyhedra, and
comes in contrast with the polynomiality of vertex enumeration for
0/1-polytopes \cite{BL98} [Bussieck and L\"ubbecke (1998)].
