diff --git a/core/CCPersistentHashtbl.ml b/core/CCPersistentHashtbl.ml
index 2f613783..71c1610c 100644
--- a/core/CCPersistentHashtbl.ml
+++ b/core/CCPersistentHashtbl.ml
@@ -26,6 +26,9 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 (** {1 Persistent hash-table on top of OCaml's hashtables} *)
 
 type 'a sequence = ('a -> unit) -> unit
+type 'a printer = Buffer.t -> 'a -> unit
+type 'a formatter = Format.formatter -> 'a -> unit
+type 'a equal = 'a -> 'a -> bool
 
 module type HashedType = sig
   type t
@@ -40,52 +43,92 @@ module type S = sig
   type 'a t
 
   val empty : unit -> 'a t
-    (** Empty table. The table will be allocated at the first binding *)
+  (** Empty table. The table will be allocated at the first binding *)
 
   val create : int -> 'a t
-    (** Create a new hashtable *)
+  (** Create a new hashtable, with the given initial capacity *)
 
   val is_empty : 'a t -> bool
-    (** Is the table empty? *)
+  (** Is the table empty? *)
 
   val find : 'a t -> key -> 'a
-    (** Find the value for this key, or raise Not_found *)
+  (** Find the value for this key, or fails
+      @raise Not_found if the key is not present in the table *)
+
+  val get_exn : key -> 'a t -> 'a
+  (** Synonym to {!find} with flipped arguments *)
+
+  val get : key -> 'a t -> 'a option
+  (** Safe version of !{get_exn} *)
 
   val mem : 'a t -> key -> bool
-    (** Is the key bound? *)
+  (** Is the key bound? *)
 
-  val length : 'a t -> int
-    (** Number of bindings *)
+  val length : _ t -> int
+  (** Number of bindings *)
 
   val replace : 'a t -> key -> 'a -> 'a t
-    (** Add the binding to the table, returning a new table. This erases
-        the current binding for [key], if any. *)
+  (** Add the binding to the table, returning a new table. This erases
+      the current binding for [key], if any. *)
+
+  val update : 'a t -> key -> ('a option -> 'a option) -> 'a t
+  (** [update tbl key f] calls [f None] if [key] doesn't belong in [tbl],
+      [f (Some v)] if [key -> v] otherwise; If [f] returns [None] then
+      [key] is removed, else it returns [Some v'] and [key -> v'] is added. *)
 
   val remove : 'a t -> key -> 'a t
-    (** Remove the key *)
+  (** Remove the key *)
 
   val copy : 'a t -> 'a t
-    (** Fresh copy of the table; the underlying structure is not shared
-        anymore, so using both tables alternatively will be efficient *)
+  (** Fresh copy of the table; the underlying structure is not shared
+      anymore, so using both tables alternatively will be efficient *)
 
-  val merge : (key -> 'a option -> 'a option -> 'a option) -> 'a t -> 'a t -> 'a t
-    (** Merge two tables together into a new table *)
+  val merge : (key -> 'a option -> 'a option -> 'a option) ->
+              'a t -> 'a t -> 'a t
+  (** Merge two tables together into a new table. The function's argument
+      correspond to values associated with the key (if present); if the
+      function returns [None] the key will not appear in the result. *)
 
   val iter : 'a t -> (key -> 'a -> unit) -> unit
-    (** Iterate over bindings *)
+  (** Iterate over bindings *)
 
   val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
-    (** Fold over bindings *)
+  (** Fold over bindings *)
 
-  val of_seq : ?init:'a t -> (key * 'a) sequence -> 'a t
-    (** Add (replace) bindings from the sequence to the table *)
+  val map : (key -> 'a -> 'b) -> 'a t -> 'b t
+  (** Map all values *)
 
-  val of_list : ?init:'a t -> (key * 'a) list -> 'a t
+  val filter : (key -> 'a -> bool) -> 'a t -> 'a t
+
+  val filter_map : (key -> 'a -> 'b option) -> 'a t -> 'b t
+
+  val for_all : (key -> 'a -> bool) -> 'a t -> bool
+
+  val exists : (key -> 'a -> bool) -> 'a t -> bool
+
+  (** {3 Conversions} *)
+
+  val of_seq : (key * 'a) sequence -> 'a t
+  (** Add (replace) bindings from the sequence to the table *)
+
+  val of_list : (key * 'a) list -> 'a t
+
+  val add_seq : 'a t -> (key * 'a) sequence -> 'a t
+
+  val add_list : 'a t -> (key  * 'a) list -> 'a t
 
   val to_seq : 'a t -> (key * 'a) sequence
-    (** Sequence of the bindings of the table *)
+  (** Sequence of the bindings of the table *)
 
   val to_list : 'a t -> (key * 'a) list
+
+  (** {3 Misc} *)
+
+  val equal : 'a equal -> 'a t equal
+
+  val pp : key printer -> 'a printer -> 'a t printer  
+
+  val print : key formatter -> 'a formatter -> 'a t formatter
 end
 
 (** {2 Implementation} *)
@@ -107,64 +150,55 @@ module Make(H : HashedType) : S with type key = H.t = struct
 
   let empty () = create 11
 
-  (** Reroot: modify the zipper so that the current node is a proper
-      hashtable, and return the hashtable *)
-  let reroot t =
-    (* pass continuation to get a tailrec rerooting *)
-    let rec reroot t k = match !t with
-    | Table tbl -> k tbl  (* done *)
-    | Add (key, v, t') ->
-      reroot t'
-        (fun tbl ->
-          t' := Remove (key, t);
-          Table.add tbl key v;
-          t := Table tbl;
-          k tbl)
-    | Replace (key, v, t') ->
-      reroot t'
-        (fun tbl ->
-          let v' = Table.find tbl key in
-          t' := Replace (key, v', t);
-          t := Table tbl;
-          Table.replace tbl key v;
-          k tbl)
-    | Remove (key, t') ->
-      reroot t'
-        (fun tbl ->
-          let v = Table.find tbl key in
-          t' := Add (key, v, t);
-          t := Table tbl;
-          Table.remove tbl key;
-          k tbl)
-    in
-    match !t with
+  (* pass continuation to get a tailrec rerooting *)
+  let rec _reroot t k = match !t with
+  | Table tbl -> k tbl  (* done *)
+  | Add (key, v, t') ->
+    _reroot t'
+      (fun tbl ->
+        t' := Remove (key, t);
+        Table.add tbl key v;
+        t := Table tbl;
+        k tbl)
+  | Replace (key, v, t') ->
+    _reroot t'
+      (fun tbl ->
+        let v' = Table.find tbl key in
+        t' := Replace (key, v', t);
+        t := Table tbl;
+        Table.replace tbl key v;
+        k tbl)
+  | Remove (key, t') ->
+    _reroot t'
+      (fun tbl ->
+        let v = Table.find tbl key in
+        t' := Add (key, v, t);
+        t := Table tbl;
+        Table.remove tbl key;
+        k tbl)
+
+  (* Reroot: modify the zipper so that the current node is a proper
+     hashtable, and return the hashtable *)
+  let reroot t = match !t with
     | Table tbl -> tbl
-    | _ -> reroot t (fun x -> x)
+    | _ -> _reroot t (fun x -> x)
 
-  let is_empty t =
-    match !t with
-    | Table tbl -> Table.length tbl = 0
-    | _ -> Table.length (reroot t) = 0
+  let is_empty t = Table.length (reroot t) = 0
 
-  let find t k =
-    match !t with
-    | Table tbl -> Table.find tbl k
-    | _ -> Table.find (reroot t) k
+  let find t k = Table.find (reroot t) k
 
-  let mem t k =
-    match !t with
-    | Table tbl -> Table.mem tbl k
-    | _ -> Table.mem (reroot t) k
+  let get_exn k t = find t k
 
-  let length t =
-    match !t with
-    | Table tbl -> Table.length tbl
-    | _ -> Table.length (reroot t)
+  let get k t =
+    try Some (find t k)
+    with Not_found -> None
+
+  let mem t k = Table.mem (reroot t) k
+
+  let length t = Table.length (reroot t)
 
   let replace t k v =
-    let tbl = match !t with
-    | Table tbl -> tbl
-    | _ -> reroot t in
+    let tbl = reroot t in
     (* create the new hashtable *)
     let t' = ref (Table tbl) in
     (* update [t] to point to the new hashtable *)
@@ -172,15 +206,14 @@ module Make(H : HashedType) : S with type key = H.t = struct
       let v' = Table.find tbl k in
       t := Replace (k, v', t')
     with Not_found ->
-      t := Remove (k, t'));
+      t := Remove (k, t')
+    );
     (* modify the underlying hashtable *)
     Table.replace tbl k v;
     t'
 
   let remove t k =
-    let tbl = match !t with
-    | Table tbl -> tbl
-    | _ -> reroot t in
+    let tbl = reroot t in
     try
       let v' = Table.find tbl k in
       (* value present, make a new hashtable without this value *)
@@ -192,26 +225,63 @@ module Make(H : HashedType) : S with type key = H.t = struct
       (* not member, nothing to do *)
       t
 
+  let update t k f =
+    let v = get k t in
+    match v, f v with
+    | None, None -> t  (* no change *)
+    | Some _, None -> remove t k
+    | _, Some v' -> replace t k v'
+
   let copy t =
-    let tbl = match !t with
-    | Table tbl -> tbl
-    | _ -> reroot t in
+    let tbl = reroot t in
     (* no one will point to the new [t] *)
     let t = ref (Table (Table.copy tbl)) in
     t
   
   let iter t f =
-    let tbl = match !t with
-    | Table tbl -> tbl
-    | _ -> reroot t in
+    let tbl = reroot t in
     Table.iter f tbl
 
   let fold f acc t =
-    let tbl = match !t with
-    | Table tbl -> tbl
-    | _ -> reroot t in
+    let tbl = reroot t in
     Table.fold (fun k v acc -> f acc k v) tbl acc
 
+  let map f t =
+    let tbl = reroot t in
+    let res = Table.create (Table.length tbl) in
+    Table.iter (fun k v -> Table.replace res k (f k v)) tbl;
+    ref (Table res)
+
+  let filter p t =
+    let tbl = reroot t in
+    let res = Table.create (Table.length tbl) in
+    Table.iter (fun k v -> if p k v then Table.replace res k v) tbl;
+    ref (Table res)
+
+  let filter_map f t =
+    let tbl = reroot t in
+    let res = Table.create (Table.length tbl) in
+    Table.iter
+      (fun k v -> match f k v with
+        | None -> ()
+        | Some v' -> Table.replace res k v'
+      ) tbl;
+    ref (Table res)
+
+  exception ExitPTbl
+
+  let for_all p t =
+    try
+      iter t (fun k v -> if not (p k v) then raise ExitPTbl);
+      true
+    with ExitPTbl -> false
+
+  let exists p t =
+    try
+      iter t (fun k v -> if p k v then raise ExitPTbl);
+      false
+    with ExitPTbl -> true
+
   let merge f t1 t2 =
     let tbl = Table.create (max (length t1) (length t2)) in
     iter t1
@@ -227,15 +297,17 @@ module Make(H : HashedType) : S with type key = H.t = struct
           | Some v' -> Table.replace tbl k v2);
     ref (Table tbl)
 
-  let of_seq ?init seq =
-    let tbl = match init with
-    | None -> Table.create 5
-    | Some t -> Table.copy (reroot t) in
-      seq (fun (k,v) -> Table.replace tbl k v);
-      ref (Table tbl)
+  let add_seq init seq =
+    let tbl = ref init in
+    seq (fun (k,v) -> tbl := replace !tbl k v);
+    !tbl
 
-  let of_list ?init l =
-    of_seq ?init (fun k -> List.iter k l)
+  let of_seq seq = add_seq (empty ()) seq
+
+  let add_list init l =
+    add_seq init (fun k -> List.iter k l)
+
+  let of_list l = add_list (empty ()) l
 
   let to_list t =
     let tbl = reroot t in
@@ -244,10 +316,36 @@ module Make(H : HashedType) : S with type key = H.t = struct
 
   let to_seq t =
     fun k ->
-      let tbl = match !t with
-      | Table tbl -> tbl
-      | _ -> reroot t
-      in
+      let tbl = reroot t in
       Table.iter (fun x y -> k (x,y)) tbl
+
+  let equal eq t1 t2 =
+    length t1 = length t2
+    &&
+    for_all
+      (fun k v -> match get k t2 with
+        | None -> false
+        | Some v' -> eq v v'
+      ) t1
+
+  let pp pp_k pp_v buf t =
+    Buffer.add_string buf "{";
+    let first = ref true in
+    iter t
+      (fun k v ->
+        if !first then first:=false else Buffer.add_string buf ", ";
+        Printf.bprintf buf "%a -> %a" pp_k k pp_v v
+      );
+    Buffer.add_string buf "}"
+ 
+  let print pp_k pp_v fmt t =
+    Format.pp_print_string fmt "{";
+    let first = ref true in
+    iter t
+      (fun k v ->
+        if !first then first:=false else Format.pp_print_string fmt ", ";
+        Format.fprintf fmt "%a -> %a" pp_k k pp_v v
+      );
+    Format.pp_print_string fmt "}"
 end
 
diff --git a/core/CCPersistentHashtbl.mli b/core/CCPersistentHashtbl.mli
index 6de2760b..30b07f4e 100644
--- a/core/CCPersistentHashtbl.mli
+++ b/core/CCPersistentHashtbl.mli
@@ -23,9 +23,19 @@ OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *)
 
-(** {1 Persistent hash-table on top of OCaml's hashtables} *)
+(** {1 Persistent hash-table on top of OCaml's hashtables}
+
+Almost as efficient as the regular Hashtbl type, but with a persistent
+interface (rewinding changes to get back in the past history). This is
+mostly useful for backtracking-like uses, or forward uses (never using
+old values).
+
+This module is not thread-safe. *)
 
 type 'a sequence = ('a -> unit) -> unit
+type 'a printer = Buffer.t -> 'a -> unit
+type 'a formatter = Format.formatter -> 'a -> unit
+type 'a equal = 'a -> 'a -> bool
 
 module type HashedType = sig
   type t
@@ -40,52 +50,92 @@ module type S = sig
   type 'a t
 
   val empty : unit -> 'a t
-    (** Empty table. The table will be allocated at the first binding *)
+  (** Empty table. The table will be allocated at the first binding *)
 
   val create : int -> 'a t
-    (** Create a new hashtable *)
+  (** Create a new hashtable, with the given initial capacity *)
 
   val is_empty : 'a t -> bool
-    (** Is the table empty? *)
+  (** Is the table empty? *)
 
   val find : 'a t -> key -> 'a
-    (** Find the value for this key, or raise Not_found *)
+  (** Find the value for this key, or fails
+      @raise Not_found if the key is not present in the table *)
+
+  val get_exn : key -> 'a t -> 'a
+  (** Synonym to {!find} with flipped arguments *)
+
+  val get : key -> 'a t -> 'a option
+  (** Safe version of !{get_exn} *)
 
   val mem : 'a t -> key -> bool
-    (** Is the key bound? *)
+  (** Is the key bound? *)
 
-  val length : 'a t -> int
-    (** Number of bindings *)
+  val length : _ t -> int
+  (** Number of bindings *)
 
   val replace : 'a t -> key -> 'a -> 'a t
-    (** Add the binding to the table, returning a new table. This erases
-        the current binding for [key], if any. *)
+  (** Add the binding to the table, returning a new table. This erases
+      the current binding for [key], if any. *)
+
+  val update : 'a t -> key -> ('a option -> 'a option) -> 'a t
+  (** [update tbl key f] calls [f None] if [key] doesn't belong in [tbl],
+      [f (Some v)] if [key -> v] otherwise; If [f] returns [None] then
+      [key] is removed, else it returns [Some v'] and [key -> v'] is added. *)
 
   val remove : 'a t -> key -> 'a t
-    (** Remove the key *)
+  (** Remove the key *)
 
   val copy : 'a t -> 'a t
-    (** Fresh copy of the table; the underlying structure is not shared
-        anymore, so using both tables alternatively will be efficient *)
+  (** Fresh copy of the table; the underlying structure is not shared
+      anymore, so using both tables alternatively will be efficient *)
 
-  val merge : (key -> 'a option -> 'a option -> 'a option) -> 'a t -> 'a t -> 'a t
-    (** Merge two tables together into a new table *)
+  val merge : (key -> 'a option -> 'a option -> 'a option) ->
+              'a t -> 'a t -> 'a t
+  (** Merge two tables together into a new table. The function's argument
+      correspond to values associated with the key (if present); if the
+      function returns [None] the key will not appear in the result. *)
 
   val iter : 'a t -> (key -> 'a -> unit) -> unit
-    (** Iterate over bindings *)
+  (** Iterate over bindings *)
 
   val fold : ('b -> key -> 'a -> 'b) -> 'b -> 'a t -> 'b
-    (** Fold over bindings *)
+  (** Fold over bindings *)
 
-  val of_seq : ?init:'a t -> (key * 'a) sequence -> 'a t
-    (** Add (replace) bindings from the sequence to the table *)
+  val map : (key -> 'a -> 'b) -> 'a t -> 'b t
+  (** Map all values *)
 
-  val of_list : ?init:'a t -> (key * 'a) list -> 'a t
+  val filter : (key -> 'a -> bool) -> 'a t -> 'a t
+
+  val filter_map : (key -> 'a -> 'b option) -> 'a t -> 'b t
+
+  val for_all : (key -> 'a -> bool) -> 'a t -> bool
+
+  val exists : (key -> 'a -> bool) -> 'a t -> bool
+
+  (** {3 Conversions} *)
+
+  val of_seq : (key * 'a) sequence -> 'a t
+  (** Add (replace) bindings from the sequence to the table *)
+
+  val of_list : (key * 'a) list -> 'a t
+
+  val add_seq : 'a t -> (key * 'a) sequence -> 'a t
+
+  val add_list : 'a t -> (key  * 'a) list -> 'a t
 
   val to_seq : 'a t -> (key * 'a) sequence
-    (** Sequence of the bindings of the table *)
+  (** Sequence of the bindings of the table *)
 
   val to_list : 'a t -> (key * 'a) list
+
+  (** {3 Misc} *)
+
+  val equal : 'a equal -> 'a t equal
+
+  val pp : key printer -> 'a printer -> 'a t printer  
+
+  val print : key formatter -> 'a formatter -> 'a t formatter
 end
 
 (** {2 Implementation} *)