diff --git a/_oasis b/_oasis
index 1f182e98..c1421116 100644
--- a/_oasis
+++ b/_oasis
@@ -41,7 +41,7 @@ Library "containers"
   Modules:          CCVector, CCDeque, CCGen, CCSequence, CCFQueue, CCMultiMap,
                     CCMultiSet, CCBV, CCPrint, CCPersistentHashtbl,
                     CCLeftistheap, CCList, CCOpt, CCPair, CCFun, CCHash,
-                    CCKList, CCInt, CCBool, CCArray, CCBatch
+                    CCKList, CCInt, CCBool, CCArray, CCBatch, CCLinq
   FindlibName:      containers
 
 Library "containers_string"
@@ -60,7 +60,7 @@ Library "containers_misc"
                     Bij, PiCalculus, Bencode, Sexp, RAL,
                     UnionFind, SmallSet, AbsSet, CSM,
                     ActionMan, QCheck, BencodeOnDisk, TTree,
-                    HGraph, Automaton, Conv, Bidir, Iteratee, Linq,
+                    HGraph, Automaton, Conv, Bidir, Iteratee,
                     Ty, Tell, BencodeStream, RatTerm, Cause, AVL, ParseReact
   BuildDepends:     unix,containers
   FindlibName:      misc
diff --git a/core/CCLinq.ml b/core/CCLinq.ml
index a2cda146..9728d414 100644
--- a/core/CCLinq.ml
+++ b/core/CCLinq.ml
@@ -30,39 +30,61 @@ type 'a sequence = ('a -> unit) -> unit
 type 'a equal = 'a -> 'a -> bool
 type 'a ord = 'a -> 'a -> int
 type 'a hash = 'a -> int
-type 'a klist = unit -> [ `Nil | `Cons of 'a * 'a klist ]
 
 let _id x = x
 
+type 'a search_result =
+  | SearchContinue
+  | SearchStop of 'a
+
 module Coll = struct
   type 'a t =
-    | Seq of 'a sequence
-    | List of 'a list
+    | Seq : 'a sequence -> 'a t
+    | List : 'a list -> 'a t
+    | Set : (module CCSequence.Set.S
+             with type elt = 'a and type t = 'b) * 'b -> 'a t
 
   let of_seq s = Seq s
   let of_list l = List l
   let of_array a = Seq (CCSequence.of_array a)
-  let empty = List []
 
-  let to_seq = function
+  let set_of_seq (type elt) ?(cmp=Pervasives.compare) seq =
+    let module S = CCSequence.Set.Make(struct
+      type t = elt
+      let compare = cmp
+    end) in
+    let set = S.of_seq seq in
+    Set ((module S), set)
+
+  let to_seq (type elt) = function
     | Seq s -> s
     | List l -> (fun k -> List.iter k l)
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        S.to_seq set
 
-  let to_list = function
+  let to_list (type elt) = function
     | Seq s -> CCSequence.to_list s
     | List l -> l
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        S.elements set
 
   let _fmap ~lst ~seq c = match c with
     | List l -> List (lst l)
     | Seq s -> Seq (seq s)
+    | Set _ ->
+        List (lst (to_list c))
 
-  let _fold ~lst ~seq acc c = match c with
-    | List l -> List.fold_left lst acc l
-    | Seq s -> CCSequence.fold seq acc s
-
-  let iter f c = match c with
-    | List l -> List.iter f l
-    | Seq s -> s f
+  let fold (type elt) f acc c = match c with
+    | List l -> List.fold_left f acc l
+    | Seq s -> CCSequence.fold f acc s
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        S.fold (fun x acc -> f acc x) set acc
 
   let map f c =
     _fmap ~lst:(List.map f) ~seq:(CCSequence.map f) c
@@ -72,16 +94,79 @@ module Coll = struct
 
   let flat_map f c =
     let c' = to_seq c in
-    Seq (CCSequence.flatMap f c')
+    Seq (CCSequence.flatMap (fun x -> to_seq (f x)) c')
 
   let filter_map f c =
     _fmap ~lst:(CCList.filter_map f) ~seq:(CCSequence.fmap f) c
 
-  let size = function
+  let size (type elt) = function
     | List l -> List.length l
     | Seq s -> CCSequence.length s
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        S.cardinal set
 
-  let fold f acc c = _fold ~lst:f ~seq:f acc c
+  let choose (type elt) = function
+    | List [] -> None
+    | List (x::_) -> Some x
+    | Seq s ->
+        begin match CCSequence.take 1 s |> CCSequence.to_list with
+        | [x] -> Some x
+        | _ -> None
+        end
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        try Some (S.choose set) with Not_found -> None
+
+  let take n c =
+    _fmap ~lst:(CCList.take n) ~seq:(CCSequence.take n) c
+
+  exception MySurpriseExit
+
+  let _seq_take_while p seq k =
+    try
+      seq (fun x -> if not (p x) then k x else raise MySurpriseExit)
+    with MySurpriseExit -> ()
+
+  let take_while p c =
+    to_seq c |> _seq_take_while p |> of_seq
+
+  let distinct ~cmp c = set_of_seq ~cmp (to_seq c)
+
+  let sort cmp c = match c with
+    | List l -> List (List.sort cmp l)
+    | _ ->
+        to_seq c |> set_of_seq ~cmp
+
+  let search obj c =
+    let _search_seq obj seq =
+      let ret = ref None in
+      begin try
+        seq (fun x -> match obj#check x with
+          | SearchContinue -> ()
+          | SearchStop y -> ret := Some y; raise MySurpriseExit);
+      with MySurpriseExit -> ()
+      end;
+      match !ret with
+      | None -> obj#failure
+      | Some x -> x
+    in
+    to_seq c |> _search_seq obj
+
+  let contains (type elt) ~eq x c = match c with
+    | List l -> List.exists (eq x) l
+    | Seq s -> CCSequence.exists (eq x) s
+    | Set (m, set) ->
+        let module S = (val m : CCSequence.Set.S
+             with type elt = elt and type t = 'b) in
+        (* XXX: here we don't use the equality relation *)
+        try
+          let y = S.find x set in
+          assert (eq x y);
+          true
+        with Not_found -> false
 end
 
 type 'a collection = 'a Coll.t
@@ -95,7 +180,14 @@ module Map = struct
     to_seq : ('a * 'b) sequence;
   }
 
-  let make_hash (type key) ?(eq=(=)) ?(hash=Hashtbl.hash) seq =
+  type ('a, 'b) build = {
+    mutable cur : ('a, 'b) t;
+    add : 'a -> 'b -> unit;
+    update : 'a -> ('b option -> 'b option) -> unit;
+  }
+
+  (* careful to use this map linearly *)
+  let make_hash (type key) ?(eq=(=)) ?(hash=Hashtbl.hash) () =
     let module H = Hashtbl.Make(struct
       type t = key
       let equal = eq
@@ -103,57 +195,99 @@ module Map = struct
     end) in
     (* build table *)
     let tbl = H.create 32 in
-    seq
-      (fun (k,v) ->
-        let l = try H.find tbl k with Not_found -> [] in
-        H.replace tbl k (v::l)
-      );
-    (* provide the multimap interface *)
-    let to_seq cont = H.iter (fun k v -> cont (k, Coll.of_list v)) tbl
-    in
-    {
+    let cur = {
       is_empty = (fun () -> H.length tbl = 0);
       size = (fun () -> H.length tbl);
       get = (fun k ->
-        try Some (Coll.of_list (H.find tbl k))
+        try Some (H.find tbl k)
         with Not_found -> None);
-      fold = (fun f acc -> H.fold (fun k v acc -> f acc k (Coll.of_list v)) tbl acc);
-      to_seq;
+      fold = (fun f acc -> H.fold (fun k v acc -> f acc k v) tbl acc);
+      to_seq = (fun k -> H.iter (fun key v -> k (key,v)) tbl);
+    } in
+    { cur;
+      add = (fun k v -> H.replace tbl k v);
+      update = (fun k f ->
+        match (try f (Some (H.find tbl k)) with Not_found -> f None) with
+        | None -> H.remove tbl k
+        | Some v' -> H.replace tbl k v');
     }
 
-  let make_map (type key) (type value)
-  ?(cmp_key=Pervasives.compare) ?(cmp_val=Pervasives.compare) seq =
+  let make_cmp (type key) ?(cmp=Pervasives.compare) () =
     let module M = CCSequence.Map.Make(struct
       type t = key
-      let compare = cmp_key
+      let compare = cmp
     end) in
-    let module S = CCSequence.Set.Make(struct
-      type t = value
-      let compare = cmp_val
-    end) in
-    let _map_set set = Coll.of_seq (S.to_seq set) in
-    let map = CCSequence.fold
-      (fun map (k,v) ->
-        let set = try M.find k map with Not_found -> S.empty in
-        M.add k (S.add v set) map
-      ) M.empty seq
-    in
-    let to_seq = 
-      M.to_seq map |> CCSequence.map (fun (k,v) -> k, _map_set v)
-    in
-    {
-      is_empty = (fun () -> M.is_empty map);
-      size = (fun () -> M.cardinal map);
+    let map = ref M.empty in
+    let cur = {
+      is_empty = (fun () -> M.is_empty !map);
+      size = (fun () -> M.cardinal !map);
       get = (fun k ->
-        try Some (_map_set (M.find k map))
+        try Some (M.find k !map)
         with Not_found -> None);
       fold = (fun f acc ->
         M.fold
-          (fun key set acc -> f acc key (_map_set set)) map acc
+          (fun key set acc -> f acc key set) !map acc
       );
-      to_seq;
+      to_seq = M.to_seq !map;
+    } in
+    {
+      cur;
+      add = (fun k v -> map := M.add k v !map);
+      update = (fun k f ->
+        match (try f (Some (M.find k !map)) with Not_found -> f None) with
+        | None -> map := M.remove k !map
+        | Some v' -> map := M.add k v' !map);
     }
 
+  type 'a key_info = {
+    eq : 'a equal option;
+    cmp : 'a ord option;
+    hash : 'a hash option;
+  }
+
+  let default_key_info = {
+    eq=None; cmp=None; hash=None;
+  }
+
+  let make_info info =
+    match info with
+    | { hash=None; _}
+    | { eq=None; _} ->
+        begin match info.cmp with
+        | None -> make_cmp ()
+        | Some cmp -> make_cmp ~cmp ()
+        end
+    | {eq=Some eq; hash=Some hash; _} -> make_hash ~eq ~hash ()
+
+  let multiset build seq =
+    seq (fun (k,v) ->
+      build.update k (function
+        | None -> Some [v]
+        | Some l -> Some (v::l)));
+    { is_empty = build.cur.is_empty;
+      size = build.cur.size;
+      get = (fun k -> match build.cur.get k with
+        | None -> None
+        | Some v -> Some (Coll.of_list v));
+      fold = (fun f acc ->
+        build.cur.fold (fun acc k v -> f acc k (Coll.of_list v)) acc);
+      to_seq = build.cur.to_seq
+        |> CCSequence.map (fun (k,v) -> k,Coll.of_list v);
+    }
+
+  let multimap_cmp ?cmp seq =
+    let build = make_cmp ?cmp () in
+    multiset build seq
+
+  let count build seq =
+    seq (fun x ->
+      let n = match build.cur.get x with
+        | None -> 1
+        | Some n -> n+1
+      in
+      build.add x n);
+    build.cur
+
   let get m x = m.get x
 
   let get_exn m x =
@@ -164,26 +298,14 @@ module Map = struct
   let size m = m.size ()
 
   let to_seq m = m.to_seq
-
-  type 'a key_info = {
-    eq : 'a equal option;
-    cmp : 'a ord option;
-    hash : 'a hash option;
-  }
 end
 
 (** {2 Query operators} *)
 
-type safe = Safe
-type unsafe = Unsafe
 type (_,_) safety =
   | Safe : ('a, 'a option) safety
   | Unsafe : ('a, 'a) safety
 
-type 'a search_result =
-  | SearchContinue
-  | SearchStop of 'a
-
 type (_, _) unary =
   | Map : ('a -> 'b) -> ('a collection, 'b collection) unary
   | GeneralMap : ('a -> 'b) -> ('a, 'b) unary
@@ -198,21 +320,21 @@ type (_, _) unary =
   | Take : int -> ('a collection, 'a collection) unary
   | TakeWhile : ('a -> bool) -> ('a collection, 'a collection) unary
   | Sort : 'a ord -> ('a collection, 'a collection) unary
-  | Distinct : 'a ord option * 'a equal option * 'a hash option
-    -> ('a collection, 'a collection) unary
+  | Distinct : 'a ord -> ('a collection, 'a collection) unary
   | Search :
     < check: ('a -> 'b search_result);
       failure : 'b;
     > -> ('a collection, 'b) unary
+  | Contains : 'a equal * 'a -> ('a collection, bool) unary
   | Get : ('b,'c) safety * 'a -> (('a,'b) Map.t, 'c) unary
-  | GroupBy : 'b ord * 'a ord * ('a -> 'b)
+  | GroupBy : 'b ord * ('a -> 'b)
     -> ('a collection, ('b,'a collection) Map.t) unary
   | Count : 'a ord -> ('a collection, ('a, int) Map.t) unary
 
 type ('a,'b,'key,'c) join_descr = {
   join_key1 : 'a -> 'key;
   join_key2 : 'b -> 'key;
-  join_merge : 'key -> 'a -> 'b -> 'c;
+  join_merge : 'key -> 'a -> 'b -> 'c option;
   join_key : 'key Map.key_info;
 }
 
@@ -234,7 +356,6 @@ type (_, _, _) binary =
   | Product : ('a collection, 'b collection, ('a*'b) collection) binary
   | Append : ('a collection, 'a collection, 'a collection) binary
   | SetOp : set_op * 'a ord -> ('a collection, 'a collection, 'a collection) binary
-  | Inter : 'a ord -> ('a collection, 'a collection, 'a collection) binary
 
 (* type of queries that return a 'a *)
 and 'a t =
@@ -258,12 +379,6 @@ let start_hashtbl h =
 let start_seq seq =
   Start (Coll.of_seq seq)
 
-(** {6 Composition} *)
-
-let apply u q = Unary (u, q)
-
-let (>>>) = apply
-
 (** {6 Execution} *)
 
 let rec _optimize : type a. a t -> a t
@@ -274,26 +389,98 @@ let rec _optimize : type a. a t -> a t
     | Binary (b, q1, q2) ->
         _optimize_binary b (_optimize q1) (_optimize q2)
     | QueryMap (f, q) -> QueryMap (f, _optimize q)
+    | Bind _ -> q  (* cannot optimize before execution *)
 and _optimize_unary : type a b. (a,b) unary -> a t -> b t
   = fun u q -> match u, q with
     | Map f, Unary (Map g, q') ->
         _optimize_unary (Map (fun x -> f (g x))) q'
+    | Filter p, Unary (Map f, cont) ->
+        _optimize_unary
+          (FilterMap (fun x -> let y = f x in if p y then Some y else None))
+          cont
+    | Map f, Unary (Filter p, cont) ->
+        _optimize_unary
+          (FilterMap (fun x -> if p x then Some (f x) else None))
+          cont
+    | Map f, Binary (Append, q1, q2) ->
+        _optimize_binary Append (Unary (u, q1)) (Unary (u, q2))
+    | Filter p, Binary (Append, q1, q2) ->
+        _optimize_binary Append (Unary (u, q1)) (Unary (u, q2))
+    | Fold (f,acc), Unary (Map f', cont) ->
+        _optimize_unary
+          (Fold ((fun acc x -> f acc (f' x)), acc))
+          cont
+    | Reduce (safety, start, mix, stop), Unary (Map f, cont) ->
+        _optimize_unary
+          (Reduce (safety,
+            (fun x -> start (f x)),
+            (fun x acc -> mix (f x) acc),
+            stop))
+          cont
+    | Size, Unary (Map _, cont) ->
+        _optimize_unary Size cont  (* ignore the map! *)
+    | Size, Unary (Sort _, cont) ->
+        _optimize_unary Size cont
     | _ -> Unary (u,q)
-    (* TODO *)
+    (* TODO: other cases *)
 and _optimize_binary : type a b c. (a,b,c) binary -> a t -> b t -> c t
   = fun b q1 q2 -> match b, q1, q2 with
-  | _ -> Binary (b, q1, q2)  (* TODO *)
-
+    | _ -> Binary (b, q1, q2)  (* TODO *)
 
 (* apply a unary operator on a collection *)
 let _do_unary : type a b. (a,b) unary -> a -> b
 = fun u c -> match u with
   | Map f -> Coll.map f c
+  | GeneralMap f -> f c
   | Filter p -> Coll.filter p c
-  | Fold (f, acc) -> Coll.fold f acc c  (* TODO: optimize *)
+  | Fold (f, acc) -> Coll.fold f acc c
+  | Reduce (safety, start, mix, stop) ->
+      let acc = Coll.to_seq c
+        |> CCSequence.fold
+          (fun acc x -> match acc with
+            | None -> Some (start x)
+            | Some acc -> Some (mix x acc)
+          ) None
+      in
+      begin match acc, safety with
+      | Some x, Safe -> Some (stop x)
+      | None, Safe -> None
+      | Some x, Unsafe -> stop x
+      | None, Unsafe -> invalid_arg "reduce: empty collection"
+      end
+  | Size -> Coll.size c
+  | Choose Safe -> Coll.choose c
+  | Choose Unsafe ->
+      begin match Coll.choose c with
+        | Some x -> x
+        | None -> invalid_arg "choose: empty collection"
+      end
+  | FilterMap f -> Coll.filter_map f c
+  | FlatMap f -> Coll.flat_map f c
+  | Take n -> Coll.take n c
+  | TakeWhile p -> Coll.take_while p c
+  | Sort cmp -> Coll.sort cmp c
+  | Distinct cmp -> Coll.distinct ~cmp c
+  | Search obj -> Coll.search obj c
+  | Get (Safe, k) -> Map.get c k
+  | Get (Unsafe, k) -> Map.get_exn c k
+  | GroupBy (cmp,f) ->
+      Coll.to_seq c
+      |> CCSequence.map (fun x -> f x, x)
+      |> Map.multimap_cmp ~cmp
+  | Contains (eq, x) -> Coll.contains ~eq x c
+  | Count cmp ->
+      Coll.to_seq c
+      |> Map.count (Map.make_cmp ~cmp ())
+
 
 (* TODO: join of two collections *)
 let _do_join ~join c1 c2 =
+  let _build = Map.make_info join.join_key in
+  assert false
+
+(* TODO *)
+let _do_group_join ~gjoin c1 c2 =
   assert false
 
 let _do_product c1 c2 =
@@ -303,7 +490,20 @@ let _do_product c1 c2 =
 let _do_binary : type a b c. (a, b, c) binary -> a -> b -> c
 = fun b c1 c2 -> match b with
   | Join join -> _do_join ~join c1 c2
+  | GroupJoin gjoin -> _do_group_join ~gjoin c1 c2
   | Product -> _do_product c1 c2
+  | Append ->
+      Coll.of_seq (CCSequence.append (Coll.to_seq c1) (Coll.to_seq c2))
+  | SetOp (Inter,cmp) ->
+      (* use a join *)
+      _do_join ~join:{
+        join_key1=_id;
+        join_key2=_id;
+        join_merge=(fun _ a b -> Some a);
+        join_key=Map.({default_key_info with cmp=Some cmp; })
+      } c1 c2
+  | SetOp (Union,cmp) -> failwith "union: not implemented"  (* TODO *)
+  | SetOp (Diff,cmp) -> failwith "diff: not implemented"  (* TODO *)
 
 let rec _run : type a. opt:bool -> a t -> a
   = fun ~opt q -> match q with
@@ -326,8 +526,6 @@ let map f q = Unary (Map f, q)
 
 let filter p q = Unary (Filter p, q)
 
-let size q = Unary (Size, q)
-
 let choose q = Unary (Choose Safe, q)
 
 let choose_exn q = Unary (Choose Unsafe, q)
@@ -346,8 +544,8 @@ let take_while p q = Unary (TakeWhile p, q)
 
 let sort ~cmp q = Unary (Sort cmp, q)
 
-let distinct ?cmp ?eq ?hash () q =
-  Unary (Distinct (cmp,eq,hash), q)
+let distinct ?(cmp=Pervasives.compare) () q =
+  Unary (Distinct cmp, q)
 
 let get key q =
   Unary (Get (Safe, key), q)
@@ -366,8 +564,8 @@ let map_to_seq_flatten q =
   in
   Unary (GeneralMap f, q)
 
-let group_by ?(cmp_key=Pervasives.compare) ?(cmp_val=Pervasives.compare) f q =
-  Unary (GroupBy (cmp_key,cmp_val,f), q)
+let group_by ?(cmp=Pervasives.compare) f q =
+  Unary (GroupBy (cmp,f), q)
 
 let count ?(cmp=Pervasives.compare) () q =
   Unary (Count cmp, q)
@@ -401,6 +599,15 @@ let max_exn q = Unary (Reduce (Unsafe, _id, Pervasives.max, _id), q)
 let min_exn q = Unary (Reduce (Unsafe, _id, Pervasives.min, _id), q)
 let average_exn q = Unary (Reduce (Unsafe, _avg_start, _avg_mix, _avg_stop), q)
 
+let is_empty q =
+  Unary (Search (object
+    method check _ = SearchStop false (* stop in case there is an element *)
+    method failure = true
+  end), q)
+
+let contains ?(eq=(=)) x q =
+  Unary (Contains (eq, x), q)
+
 let for_all p q =
   Unary (Search (object
     method check x = if p x then SearchContinue else SearchStop false
diff --git a/core/CCLinq.mli b/core/CCLinq.mli
index b9915cc3..885c8840 100644
--- a/core/CCLinq.mli
+++ b/core/CCLinq.mli
@@ -113,8 +113,7 @@ val take_while : ('a -> bool) -> 'a collection t -> 'a collection t
 val sort : cmp:'a ord -> 'a collection t -> 'a collection t
 (** Sort items by the given comparison function *)
 
-val distinct : ?cmp:'a ord -> ?eq:'a equal -> ?hash:'a hash ->
-               unit -> 'a collection t -> 'a collection t
+val distinct : ?cmp:'a ord -> unit -> 'a collection t -> 'a collection t
 (** Remove duplicate elements from the input collection.
     All elements in the result are distinct. *)
 
@@ -135,7 +134,7 @@ val map_to_seq_flatten : ('a,'b collection) Map.t t -> ('a*'b) collection t
 
 (** {6 Aggregation} *)
 
-val group_by : ?cmp_key:'b ord -> ?cmp_val:'a ord ->
+val group_by : ?cmp:'b ord ->
                ('a -> 'b) -> 'a collection t -> ('b,'a collection) Map.t t
 (** [group_by f] takes a collection [c] as input, and returns
     a multimap [m] such that for each [x] in [c],
@@ -163,8 +162,12 @@ val reduce_exn : ('a -> 'b) -> ('a -> 'b -> 'b) -> ('b -> 'c) ->
 (** Same as {!reduce} but fails on empty collections.
     @raise Invalid_argument if the collection is empty *)
 
+val is_empty : 'a collection t -> bool t
+
 val sum : int collection t -> int t
 
+val contains : ?eq:'a equal -> 'a -> 'a collection t -> bool t
+
 val average : int collection t -> int option t
 val max : int collection t -> int option t
 val min : int collection t -> int option t
@@ -182,13 +185,14 @@ val find_map : ('a -> 'b option) -> 'a collection t -> 'b option t
 
 val join : ?cmp:'key ord -> ?eq:'key equal -> ?hash:'key hash ->
             ('a -> 'key) -> ('b -> 'key) ->
-            merge:('key -> 'a -> 'b -> 'c) ->
+            merge:('key -> 'a -> 'b -> 'c option) ->
             'a collection t -> 'b collection t -> 'c collection t
 (** [join key1 key2 ~merge] is a binary operation
     that takes two collections [a] and [b], projects their
     elements resp. with [key1] and [key2], and combine
     values [(x,y)] from [(a,b)] with the same [key]
-    using [merge]. *)
+    using [merge]. If [merge] returns [None], the combination
+    of values is discarded. *)
 
 val group_join : ?cmp:'a ord -> ?eq:'a equal -> ?hash:'a hash ->
                   ('b -> 'a) -> 'a collection t -> 'b collection t ->