anchor.diff:  Performace suggests how powerful the endpoint is.
   
   We measure the response times of the two queries, Listing
  -<<query_ask>>, <<query_for_list_of_graphs>>. The former query is a
  +<<query_ask, 15>>, <<query_for_list_of_graphs, 16>>. The former query is a
   most simple query and we use this query to estimate the transfer time.
   The latter query requires a little computations for endpoints. We
   believe the execution cost of this query does not differ very much
   according to the size of data.
   
   [[query_ask]]
  -.A Most Simple Query
  +.Listing 15. A Most Simple Query
   ....
   ASK {}
   
   ....
   
   [[query_for_list_of_graphs]]
  -.A Query for Listing Graphs
  +.Listing 16. A Query for Listing Graphs
   ....
   SELECT DISTINCT
     ?g
  
positive.code: +
  When the number of statements is not changed, we retrieve the first and
  the last statements using queries described in Listing
  <<query_for_the_first_statement>> and
  <<query_for_the_last_statement>>. Then we compare them with those of
  a previous day. If one of them is different, we assume the endpoint
  seems to be updated today.
  +
  [[query_for_the_first_statement]]
  .A query for the first statement
  ....
  SELECT *
  WHERE {?s ?p ?o}
  
negative.code: ....
  +
  [[query_for_classes_having_instances_on_a_graph]]
  .Listing 6. Obtain the classes having instances on a graph g
  ....
  PREFIX rdf:
  SELECT DISTINCT ?c
  