anchor.diff:                  2 -- 4
                   3 -- 4
               }"
  -            let g = load_undirected_test_graph
  -            let v = Visualisation.toDotGraphDescriptionLanguage g
  -            checkDotLanguage v expected            
  +            trial {
  +                let! g = load_undirected_test_graph
  +                return Visualisation.toDotGraphDescriptionLanguage g
  +            }
  +            |> returnOrFail |> checkDotLanguage expected
   
           testCase "directed graph" <| fun _ ->
               let expected = "digraph {
  
positive.code: 
      let load_undirected_test_graph = 
          let isDirected = false
          Generation.readGraph (test_graph_file "undirected_graph.txt") isDirected
  
      let load_directed_test_graph = 
          let isDirected = true
          Generation.readGraph (test_graph_file "directed_graph.txt") isDirected
  
      let neighbours graph vertexIdNumber = 
          VertexId vertexIdNumber 
          |> Graph.vertexFromId graph 
          |> (fun v -> v.Neighbours)
  
      let neighbourIdNumbers graph vertexIdNumber = 
          neighbours graph vertexIdNumber
          |> Seq.map (fun vId -> vId.Id)
          |> Array.ofSeq
          |> Array.sort
  
      let inline factorial (n: int): bigint = 
          if n < 0 then 
              failwith "negative faculty"  
  
negative.code:                 5 -> 3
                  5 -> 4
              }"        
              trial {
                  let! g = load_directed_test_graph
                  return Visualisation.toDotGraphDescriptionLanguage g
              }
              |> returnOrFail |> checkDotLanguage expected
      ]
  
  
  