+ HashSet as Monoid

Author: gusty

src/FSharpPlus/Control/Monoid.fs

@@ -29,6 +29,7 @@ type Plus =
     static member        ``+`` (x: Set<_>            , y                    , [<Optional>]_mthd: Plus    ) = Set.union x y
 
     #if !FABLE_COMPILER
+    static member        ``+`` (x: HashSet<_>        , y                    , [<Optional>]_mthd: Plus    ) = HashSet.union x y
     static member        ``+`` (x: StringBuilder     , y: StringBuilder     , [<Optional>]_mthd: Plus    ) = StringBuilder().Append(x).Append(y)
     static member        ``+`` (_: Id0               , _: Id0               , [<Optional>]_mthd: Plus    ) = Id0 ""    
     static member        ``+`` (x: AggregateException, y: AggregateException, [<Optional>]_mthd: Plus    ) = Exception.add x y

src/FSharpPlus/Control/Numeric.fs

@@ -6,6 +6,7 @@ namespace FSharpPlus.Control
 
 open System.Runtime.InteropServices
 open FSharpPlus.Internals
+open FSharpPlus
 #if FABLE_COMPILER
 /// NOTE
 type OptionalAttribute ()=
@@ -206,14 +207,15 @@ type Zero with
     static member inline Zero (_: Lazy<'a>                  , _: Zero) = let (v: 'a) = Zero.Invoke () in lazy v
     static member        Zero (_: Dictionary<'a,'b>         , _: Zero) = Dictionary<'a,'b> ()
     static member        Zero (_: ResizeArray<'a>           , _: Zero) = ResizeArray () : ResizeArray<'a>
+    static member        Zero (_: HashSet<'a>                    , _: Zero    ) = HashSet.empty : HashSet<'a>
 
 type Zero with
     static member inline Zero (_: ^R                             , _: Default6) = FromInt64.Invoke 0L : ^R
 
     static member inline Zero (_: ^R                             , _: Default5) = Implicit.Invoke 0   : ^R
 
     static member        Zero (_: seq<'a>                        , _: Default4) = Seq.empty      : seq<'a>
-    static member        Zero (_: IEnumerator<'a>                , _: Default4) = FSharpPlus.Enumerator.Empty () : IEnumerator<'a>
+    static member        Zero (_: IEnumerator<'a>                , _: Default4) = Enumerator.Empty () : IEnumerator<'a>
     static member        Zero (_: IDictionary<'a,'b>             , _: Default4) = Dictionary<'a,'b> () :> IDictionary<'a,'b>
     static member        Zero (_: IReadOnlyDictionary<'a,'b>     , _: Default4) = Dictionary<'a,'b> () :> IReadOnlyDictionary<'a,'b>
     static member inline Zero (_: 't                             , _: Default3) = (^t : (static member Empty: ^t) ()) : 't