   
   
   
+  /// the default graph is empty and non-oriented
+  
   init()
   {
     self.vertices = SPTVertice[]()

  
  
  
  // return true if there is an egde between x and y
  
  func adjacent(x: SPTVertice, y: SPTVertice) -> Bool
  {

  }
  
  
  
  // 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[]
  {
