Enum.fs

    |Shape.Enum s->
         s.Accept {
                new IEnumVisitor<Convert<'T>> with
                member __.Visit<'t ,'u  when 't : enum<'u>
                                        and 't : struct
                                        and 't :> ValueType
                                        and 't : (new : unit -> 't)>() =
                    let printer=
                      fun x->
                        let ts:'u = unbox<'t> x|>LanguagePrimitives.EnumToValue
                        ts|>BsonValue

                    let parser=fun(v:BsonValue)->
                       let res:'t=LanguagePrimitives.EnumOfValue (unbox<'u> v.AsInt32)
                       res

                    mkParser printer parser
                    }