anchor.diff:          EncodeUnionTag : System.Type -> int -> option<string * Encoded>
           /// Get the JSON-encoded name of the given F# union case field.
           GetEncodedUnionFieldName : System.Reflection.PropertyInfo -> int -> string
  +        /// If true, represent fields whose type is a union marked
  +        /// UseNullAsTrueValue as absent field.
  +        /// Also always represent options as if they were marked [<OptionalField>].
  +        RepresentNullUnionsAsAbsentField : bool
           /// Pack an encoded value to JSON.
           Pack : Encoded -> Value
       }
  
positive.code:             GetUnionTag = defaultGetUnionTag
              EncodeUnionTag = defaultEncodeUnionTag
              GetEncodedUnionFieldName = fun _ i -> "$" + string i
              Pack = pack
          }
  
  
negative.code:             match x with
              | None -> none
              | Some v -> some [| dec v |]
      elif isNullableUnion i mt then
          let dec = dD mt
          let ucis = FST.GetUnionCases(mt, flags)
          let none = FSV.PreComputeUnionConstructor(ucis.[0], flags) [||]
          function
          | Some v -> dec v
          | None -> none
      else
          let dec = dD mt
          function
  