   }
   
   
+  
+  // Return all vertices y such as
+  //
+  //  - there is an edge from x to y or 
+  //  - there is an edge from y to x.
   
   func neighbors(x: SPTVertice) -> SPTVertice[]
   {

  
  
  
  // the order of a graph is |V| (the number of vertices)
  
  func order() -> Int
  {

  
  
  
  /// return the graph's size, |E|, i.e. the number of edges
  
  func size() -> Int
  {
    return self.edges.count
  }
  
  
  /// Add the given vertice to the graph
  
  func addVertice(x: SPTVertice)
  {
