anchor.diff:  
   [<Test>]
   let ``el is equivalent to List.nth``() =
  -    assertProp "chop_list" <| fun n xs ->
  +    assertProp "el" <| fun n xs ->
           (n >= 0 && List.length xs > n) ==> 
               lazy (el n xs = List.nth xs n)
   
  
positive.code: 
  [<Test>]
  let ``end_itlist is equivalent to List.reduceBack on non-empty lists``() =
      assertProp "end_itlist" <| fun (xs : int list) f ->
          xs <> [] ==> lazy (end_itlist f xs = List.reduceBack f xs)
  
  [<Test>]
  
negative.code: 
  [<Test>]
  let ``flat is equivalent to List.concat``() =
      assertProp "flat" <| fun (xss : string list list) ->
          flat xss = List.concat xss
  
  [<Test>]
  