code: func InDegree(nodes []Node) map[Node]int {
  	degree := make(map[Node]int, len(nodes))
  	for _, n := range nodes {
  		if _, ok := degree[n]; !ok {
  			degree[n] = 0
  		}
  		for _, e := range n.Edges() {
  			degree[e.Tail()]++
  		}
  	}
  	return degree
  }
docs: // InDegree is used to compute the in-degree of nodes