Before revision: The Vertical constraint graph is the (undirected) graph with vertex set N and two nets are connected by an edge if and only if there are two pins from different nets on the same vertical line and the edge is directed from the net with pin on the upper edge of the channel.
After revision: The Vertical constraint graph is the directed graph with vertex set N and two nets are connected by an edge if and only if there are two pins from different nets on the same vertical line and the edge is directed from the net with pin on the upper edge of the channel.