Method firstIn

ot/lp/full_bipartitegraph.h:121–126

    }

    void firstIn(Arc& arc, const Node& node) const {
		if (node<_n1)
			arc = -1;
		else
			arc = _arc_num + node - _node_num;
    }

    void nextIn(Arc& arc) const {
      arc -= _n2;