class Molinillo::DependencyGraph::AddVertex

def up(graph)
  if existing = graph.vertices[name]
    @existing_payload = existing.payload
    @existing_root = existing.root
  end
  vertex = existing || Vertex.new(name, payload)
  graph.vertices[vertex.name] = vertex
  vertex.payload ||= payload
  vertex.root ||= root
  vertex
end