fix GCI

Author: akrzemi1

doc/tutorial/graph_container_interface.md

@@ -4,21 +4,54 @@ Graph Container Interface
 Generic algorithms and views in this library require that your graph container 
 is represented as an [adjacency list](https://en.wikipedia.org/wiki/Adjacency_list).
 Your container is never accessed directly. Instead, the access is performed via the 
-*graph container interface*.
+*graph container interface* (GCI).
 
 
 ```c++
-  std::vector<std::vector<int>> g {  //  
-    /*0*/ {1},                       //
-    /*1*/ {0}                        //      (0) ----- (1)
-  };    
+  std::vector<std::vector<int>> g {      //  
+    /*0*/ {1},                           //
+    /*1*/ {0}                            //      (0) ----- (1)
+  };                                     //
 
-  auto && vtcs = graph::vertices(g);    // get the std::range representing vertices
+  auto && vtcs = graph::vertices(g);     // get the std::range representing graph vertices
+  
   auto vit = std::ranges::begin(vtcs);
-  int id0 = graph::vertex_id(g, vit);   // get the id of the vertex
+  
+  int id0 = graph::vertex_id(g, vit);    // get the id of the vertex
   assert(id0 == 0);
-  auto && edgs = graph::edges(g, *vit); // 
+  
+  auto && edgs = graph::edges(g, *vit);  // get the std::range representing the neighbors of the vertex
+  
   auto eit = std::ranges::begin(edgs);
-  int id1 = graph::target_id(g, *eit);
+  
+  int id1 = graph::target_id(g, *eit);   // get the id of the vertex on the other side of the edge
   assert(id1 == 1);
-```
+  
+  auto wit = graph::find_vertex(g, id1); // get iterator to vertex based on the vertex id
+  
+  int id1_ = graph::vertex_id(g, wit);
+  assert(id1_ == 1);
+```
+
+This set of operations may seem too rich: you need fewer operations to achieve the same 
+for type `vector<vector<int>>`. But the Graph Container Interface needs to be prepared for
+very different representations of an adjacency list.
+
+The list of all CPOs in the Graph Container interface is bigger. 
+All the customization points are described in detail in section [Customization Points](../reference/customization_points.md). 
+
+
+Testing the customization
+-------------------------
+
+In order to test if your type has the necessary customization for all the CPOs, you can use concepts defined in this library.
+
+The one that you will need most often is `graph::index_adjacency_list`:
+
+```c++
+#include <graph/graph.hpp>
+
+static_assert(graph::index_adjacency_list<MyType>); 
+```
+
+In practice, you need to customize only a handful of CPOs. Most of the others have their default customizaiton.