anchor.diff:      /// <returns></returns>
       [<CompiledName("IsError")>]
       let inline isError (value : Choice<'T, 'Error>) : bool =
  +        // Preconditions
  +        checkNonNull "value" value
  +
           match value with
           | Choice1Of2 _ -> false
           | Choice2Of2 _ -> true
  
positive.code:     /// <returns></returns>
      [<CompiledName("Iterate")>]
      let iter (action : 'T -> unit) (value : Choice<'T, 'Error>) : unit =
          match value with
          | Choice2Of2 _ -> ()
          | Choice1Of2 result ->
  
negative.code:     /// <returns></returns>
      [<CompiledName("Forall")>]
      let forall (predicate : 'T -> bool) (value : Choice<'T, 'Error>) : bool =
          // Preconditions
          checkNonNull "value" value
  
          match value with
          | Choice1Of2 result ->
              predicate result
  