code: func (g *UndirectedGraph) EdgeBetween(x, y graph.Node) graph.Edge {
  	// We don't need to check if neigh exists because
  	// it's implicit in the edges access.
  	if !g.Has(x) {
  		return nil
  	}
  
  	edge, _ := g.edges[x.ID()].Get(y.ID())
  	return edge
  }
docs: // EdgeBetween returns the edge between nodes x and y.