anchor.diff:  +
   We assume all subjects of statements are things. We search invalid
   statement using a query described in Listing
  -<<non_http_uri_subject, 11>>, and if nothing is found the endpoint
  +<<non_http_uri_subject, 10>>, and if nothing is found the endpoint
   satisfied this rule.
   +
   Note that we ignore Virtuoso specific graphs since Virtuoso contains a
   graph which contains invalid statements.
   +
   [[non_http_uri_subject]]
  -.Listing 11. A Query for searching non-HTTP-URI subjects
  +.Listing 10. A Query for searching non-HTTP-URI subjects
   ....
   SELECT
     *
  
positive.code: ....
  +
  [[query_for_see_also]]
  .Listing 14. A Query for a See Also Statement
  ....
  PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
  SELECT
  
negative.code: Performace suggests how powerful the endpoint is.
  
  We measure the response times of the two queries, Listing
  <<query_ask, 14>>, <<query_for_list_of_graphs, 15>>. 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]]
  .Listing 14. A Most Simple Query
  ....
  ASK {}
  
  ....
  
  [[query_for_list_of_graphs]]
  .Listing 14. A Query for Listing Graphs
  ....
  SELECT DISTINCT
    ?g
  