anchor.diff:  procedure:
   +
   First, we retrieve a number of statements using a query described in
  -Listing <<query_for_number_of_statements>> and compare the number
  +Listing <<query_for_number_of_statements, 1>> and compare the number
   with the previous one. If the number of statements has changed, we
   assume the endpoint seems to be updated today.
   +
   [[query_for_number_of_statements]]
  -.A query for retrieving a number of statements
  +.Listing 1. A query for retrieving a number of statements
   ....
   SELECT COUNT(*) AS ?c
   WHERE {?s ?p ?o}
  
positive.code: +
  We assume all subjects of statements are things. We search invalid
  statement using a query described in Listing
  <<non_http_uri_subject>>, 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]]
  .A Query for searching non-HTTP-URI subjects
  ....
  SELECT
    *
  
negative.code: 4.  Properties
  +
  We retrieve a list of properties using a query described in Listing
  <<query_for_properties_on_a_graph, 9>>.
  +
  [[query_for_properties_on_a_graph]]
  .Listing 9. Obtain the properties on a graph g
  ....
  SELECT DISTINCT ?p
          FROM <g>
  