anchor.diff:                  (expected : _ seq)
                   format =
     let extra, missing =
  -    except(expected, actual),
  -    except(actual, expected)
  -
  -  if List.isEmpty missing then () else
  +    except(actual, expected),
  +    except(expected, actual)
  +  let isCorrect = List.isEmpty missing && List.isEmpty extra
  +  if isCorrect then () else
   
     sprintf "%s.
       Sequence `actual` does not contain all `expected` elements.
  
positive.code:       differNrOfElement element
      else Array.create (snd element) (fst element) |> Array.toList
    
    sequence
    |> groupByOccurances
    |> Seq.map elementsWhichDiffer
    |> List.concat
  
negative.code:     |> String.concat ", "
      |> sprintf "{%s}"
  
  let inline private except(baseSeq: 'a seq, exceptSeq: 'a seq): 'a list =
    let groupByOccurances sequence =
      sequence
      |> Seq.groupBy id
  