anchor.diff:        differNrOfElement element
       else Array.create (snd element) (fst element) |> Array.toList
     
  -  sequence
  +  baseSeq
     |> groupByOccurances
     |> Seq.map elementsWhichDiffer
     |> List.concat
  
positive.code:     |> String.concat ", "
      |> sprintf "{%s}"
  
  let inline private except(sequence: 'a seq, exceptSeq: 'a seq): 'a list =
    let groupByOccurances sequence =
      sequence
      |> Seq.groupBy id
  
negative.code:                 (expected : _ seq)
                  format =
    let extra, missing =
      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.
  