code: func (g *SimpleGraph) Degree(v interface{}) (int, error) {
  	g.mutex.RLock()
  	defer g.mutex.RUnlock()
  
  	val, ok := g.adjacencyList[v]
  	if !ok {
  		return 0, ErrVertexNotFound
  	}
  	return len(val), nil
  }
docs: // Degree returns the number of vertices connected to v