code: func newGraph() *graph {
  	return &graph{
  		idToNodes:     make(map[ID]Node),
  		nodeToSources: make(map[ID]map[ID]float64),
  		nodeToTargets: make(map[ID]map[ID]float64),
  		//
  		// without this
  		// panic: assignment to entry in nil map
  	}
  }
docs: // newGraph returns a new graph.