anchor.diff:  
   [<LangTheory; CSharpAndFSharp>]
   let ``Applies custom formatter to enums`` (lang) =
  -    let provider = (SizeFormatter CultureInfo.InvariantCulture) :> IFormatProvider
  +    let provider = (BernieSandersSizeFormatter CultureInfo.InvariantCulture) :> IFormatProvider
       let template = "Size {size}"
       let values : obj[] = [| Size.Large |]
  -    let expected = "Size HUUGE"
  +    let expected = "Size YUUUGE"
       MtAssert.RenderedAs(lang, template, values, expected, provider)
positive.code:     let destr = createCustomDestructurer None (Some builtInFSharpTypesDestructurer)
      MtAssert.RenderedAs(lang, template, values, expected, provider, additionalDestrs=[destr])
  
  type Size = Large = 1
  type SizeFormatter(innerFormatProvider : IFormatProvider) =
    interface IFormatProvider with
      member this.GetFormat ty =
        match ty with
  
negative.code:     member this.Format (format : string, arg : obj, provider : IFormatProvider) =
        match arg with
        | :? Size as s ->
          match s with Size.Large -> "YUUUGE" | _ -> sprintf "%A" s
        | :? IFormattable as f ->
          f.ToString(format, innerFormatProvider)
        | _ ->
  