[TESTS][LAMBDA] Add a test; small fix in GRAMMAR

tests/lambda/GRAMMAR

@@ -12,6 +12,7 @@ expr = id
 | expr "*" expr
 | expr "/" expr
 | expr "%" expr
+| expr "@" expr
 | expr "{" UIDENT "/" type_id sigma "}" (* type substitutions *)
 | "(" expr ")"
 | "[" "]" (* nil *)

tests/lambda/src/main.ml

@@ -350,6 +350,13 @@ let tests_eval = "CDuce runtime tests (Typed -> Lambda -> Value)" >:::
                              | (el : 'A{A/Int; A/Bool}) :: (rest : ['A{A/Int; A/Bool}]) -> f.rest + 1)
                         in
                         (length.[1; 2; 5; 4; 8; 7], length.[], length.[`true; 2]) : (Int*Int*Int)");
+      assert_equal ~msg:"Test CDuce.runtime.list.nth failed"
+	~printer:(fun x -> x) "5"
+	(run_test_eval "(fun nth l : ['A{A/Int; A/Bool}] n : Int : 'A{A/Int; A/Bool} ->
+                           match l : ['A{A/Int; A/Bool}] with
+                             | (el : 'A{A/Int; A/Bool}) :: [] -> el
+                             | (el : 'A{A/Int; A/Bool}) :: (rest : ['A{A/Int; A/Bool}]) ->
+                               (match n : Int with | 0 -> el | _ : Int -> (nth.rest).(n-1))).[1; 2; 5; `true; 2].2");
     );
 
     "union" >:: ( fun test_ctxt ->