Fix Appveyor error

Author: moh-hassan

src/CommandLine/Infrastructure/CSharpx/Either.cs

@@ -272,7 +272,7 @@ public static void Match<TLeft, TRight>(this Either<TLeft, TRight> either, Actio
         #endregion
 
         /// <summary>
-        /// Equivalent to monadic <see cref="CSharpx.Either.Return{TLeft, TRight}"/> operation.
+        /// Equivalent to monadic <see cref="CSharpx.Either.Return{TRight}"/> operation.
         /// Builds a <see cref="CSharpx.Right{TLeft, TRight}"/> value in case <paramref name="value"/> by default.
         /// </summary>
         public static Either<string, TRight> ToEither<TRight>(this TRight value)
@@ -312,4 +312,4 @@ public static bool IsRight<TLeft, TRight>(this Either<TLeft, TRight> either)
             return either.Tag == EitherType.Right;
         }
     }
-}
+}

tests/CommandLine.Tests/Unit/UnParserExtensionsTests.cs

@@ -6,7 +6,10 @@
 using Xunit;
 using FluentAssertions;
 using CommandLine.Tests.Fakes;
-
+#if !SKIP_FSHARP
+using Microsoft.FSharp.Core;
+#endif
+ 
 namespace CommandLine.Tests.Unit
 {
     public class UnParserExtensionsTests