bugfix in Gen; make API change explicit for Gen.merge

gen.ml

@@ -955,7 +955,7 @@ let product gena genb =
     | `Stop -> None
     | `GetLeft ->
       begin match gena() with
-        | None -> cur := `GetRight
+        | None -> cur := `GetRightOrStop
         | Some a -> all_a := a :: !all_a; cur := `ProdLeft (a, !all_b)
       end;
       next ()

gen.mli

@@ -192,11 +192,11 @@ module type S = sig
 
   val merge : 'a gen t -> 'a t
     (** Pick elements fairly in each sub-generator. The merge of enums
-        [e1, e2, ... en] picks one element in [e1], then one element in [e2],
-        then in [e3], ..., then in [en], and then starts again at [e1]. Once
-        a generator is empty, it is skipped; when they are all empty,
+        [e1, e2, ... ] picks elements in [e1], [e2],
+        in [e3], [e1], [e2] .... Once a generator is empty, it is skipped;
+        when they are all empty, and none remains in the input,
         their merge is also empty. 
-        For instance, [merge [1;3;5] [2;4;6]] will be [1;2;3;4;5;6]. *)
+        For instance, [merge [1;3;5] [2;4;6]] will be, in disorder, [1;2;3;4;5;6]. *)
 
   val intersection : ?cmp:('a -> 'a -> int) -> 'a t -> 'a t -> 'a t
     (** Intersection of two sorted sequences. Only elements that occur in both

tests/test_gen.ml

@@ -56,7 +56,7 @@ let test_filterMap () =
 let test_merge () =
   let e = Gen.of_list [1--3; 4--6; 7--9] in
   let e' = Gen.merge e in
-  OUnit.assert_equal [1;4;7;2;5;8;3;6;9] (Gen.to_list e');
+  OUnit.assert_equal [1;2;3;4;5;6;7;8;9] (Gen.to_list e' |> List.sort compare);
   ()
 
 let test_persistent () =