anchor.diff:      /// <returns>
       ///     The number of elements in <paramref name="_arg1" />.
       /// </returns>
  -    let length mset =
  -        mset |> toFlatSeq |> Seq.length
  +    let length (MSet mset) =
  +        mset |> Map.fold (fun count _ n -> count + n) 0
   
       /// <summary>
       ///     Appends two multisets.
  
positive.code:     /// <returns>
      ///     A singleton multiset.
      /// </returns>
      let singleton x = ofFlatList [ x ]
  
  
      (*
  
negative.code:     ///     A multiset containing the given items.
      /// </returns>
      let ofFlatSeq xs =
          Seq.fold add empty xs
  
      /// <summary>
      ///     Creates a multiset with the given flat list as its contents.
  