add several functions: product2, find, mem;

fix doc error on product

sequence.ml

@@ -304,12 +304,15 @@ let sort_uniq ?(cmp=Pervasives.compare) seq =
   uniq ~eq:(fun x y -> cmp x y = 0) seq'
 
 (** Cartesian product of the sequences. *)
-let product outer inner =
+let product outer inner k =
-  let inner = persistent inner in
-  from_iter
-    (fun k ->
   outer (fun x ->
-        inner (fun y -> k (x,y))))
+    inner (fun y -> k (x,y))
+  )
+
+let product2 outer inner k =
+  outer (fun x ->
+    inner (fun y -> k x y)
+  )
 
 (** [join ~join_row a b] combines every element of [a] with every
     element of [b] using [join_row]. If [join_row] returns None, then
@@ -394,6 +397,19 @@ let exists p seq =
     false
   with ExitSequence -> true
 
+let mem ?(eq=(=)) x seq = exists (eq x) seq
+
+let find f seq =
+  let r = ref None in
+  begin try
+    seq (fun x -> match f x with
+      | None -> ()
+      | Some _ as res -> r := res
+    );
+  with ExitSequence -> ()
+  end;
+  !r
+
 (** How long is the sequence? *)
 let length seq =
   let r = ref 0 in

sequence.mli

@@ -120,6 +120,15 @@ val for_all : ('a -> bool) -> 'a t -> bool
 val exists : ('a -> bool) -> 'a t -> bool
   (** Exists there some element satisfying the predicate? *)
 
+val mem : ?eq:('a -> 'a -> bool) -> 'a -> 'a t -> bool
+  (** Is the value a member of the sequence?
+      @param eq the equality predicate to use (default [(=)])
+      @since NEXT_VERSION *)
+
+val find : ('a -> 'b option) -> 'a t -> 'b option
+  (** Find the first element on which the function doesn't return [None]
+      @since NEXT_VERSION *)
+
 val length : 'a t -> int
   (** How long is the sequence? Forces the sequence. *)
 
@@ -192,9 +201,14 @@ val uniq : ?eq:('a -> 'a -> bool) -> 'a t -> 'a t
       like [fun seq -> map List.hd (group seq)]. *)
 
 val product : 'a t -> 'b t -> ('a * 'b) t
-  (** Cartesian product of the sequences. The first one is transformed
+  (** Cartesian product of the sequences. When calling [product a b],
-      by calling [persistent] on it, so that it can be traversed
+      the caller {b MUST} ensure that [b] can be traversed as many times
-      several times (outer loop of the product) *)
+      as required (several times), possibly by calling {!persistent} on it
+      beforehand. *)
+
+val product2 : 'a t -> 'b t -> ('a, 'b) t2
+  (** Binary version of {!product}. Same requirements.
+      @since NEXT_VERSION *)
 
 val join : join_row:('a -> 'b -> 'c option) -> 'a t -> 'b t -> 'c t
   (** [join ~join_row a b] combines every element of [a] with every