diff --git a/src/data/CCHashTrie.ml b/src/data/CCHashTrie.ml
index ad1d6c8f..337f268c 100644
--- a/src/data/CCHashTrie.ml
+++ b/src/data/CCHashTrie.ml
@@ -9,7 +9,7 @@ type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
 
 (** {2 Fixed-Size Arrays} *)
 module type FIXED_ARRAY = sig
-  type 'a t
+  type +'a t
   val create : 'a -> 'a t
   val length_log : int
   val length : int  (* 2 power length_log *)
@@ -68,29 +68,46 @@ end
 (** {2 Arrays} *)
 
 module A32 : FIXED_ARRAY = struct
-  type 'a t = 'a array
+  type +'a t = { dummy1: 'a; dummy2 : 'a }  (* used for variance only *)
+
+  (* NOTE for safety:
+    
+    the array and the record are both boxed types, in the heap
+    (since it has two fields it should not change in the future).
+
+    using an array as covariant is safe because we ALWAYS copy before writing,
+    so we cannot put a wrong value in [a] by upcasting it and writing.
+  *)
+
+  external hide_array_ : 'a array -> 'a t = "%identity"
+  external get_array_ : 'a t -> 'a array = "%identity"
 
   let length_log = 5
 
   let length = 1 lsl length_log  (* 32 *)
 
-  let create x = Array.make length x
+  let create x = hide_array_ (Array.make length x)
 
-  let get a i = a.(i)
+  let get a i = Array.get (get_array_ a) i
 
   let set a i x =
-    let a' = Array.copy a in
+    let a' = Array.copy (get_array_ a) in
     a'.(i) <- x;
-    a'
+    hide_array_ a'
 
   let update a i f =
-    let x = a.(i) in
-    let y = f a.(i) in
-    if x==y then a else set a i y
+    let x = Array.get (get_array_ a) i in
+    let y = f x in
+    if x==y then a
+    else (
+      let a' = Array.copy (get_array_ a) in
+      a'.(i) <- y;
+      hide_array_ a'
+    )
 
-  let iter = Array.iter
+  let iter f a = Array.iter f (get_array_ a)
 
-  let fold = Array.fold_left
+  let fold f acc a = Array.fold_left f acc (get_array_ a)
 end
 
 (** {2 Functors} *)
@@ -105,11 +122,15 @@ module Make(Key : KEY)
   module Hash : sig
     type t = private int
     val make : Key.t -> t
+    val zero : t (* special "hash" *)
+    val is_0 : t -> bool
     val rem : t -> int (* [A.length_log] last bits *)
     val quotient : t -> t (* remove [A.length_log] last bits *)
   end = struct
     type t = int
     let make = Key.hash
+    let zero = 0
+    let is_0 h = h==0
     let rem h = h land (A.length - 1)
     let quotient h = h lsr A.length_log
   end
@@ -126,13 +147,20 @@ module Make(Key : KEY)
   type 'a t =
     | E
     | L of Hash.t * 'a leaf (* same hash for all elements *)
-    | N of 'a t A.t
+    | N of 'a leaf * 'a t A.t  (* leaf for hash=0, subnodes *)
 
   (* invariants:
       L [] --> E
       N [E, E,...., E] -> E
   *)
 
+  (* NOTE for safety:
+    
+    only allocate one empty array. It will contain only [E] for every
+    different value type
+  *)
+  let empty_arr_ = A.create E
+
   let empty = E
 
   let is_empty = function
@@ -153,10 +181,12 @@ module Make(Key : KEY)
   let rec get_exn_ k ~h m = match m with
     | E -> raise Not_found
     | L (_, l) -> get_exn_list_ k l
-    | N a ->
-        let i = Hash.rem h in
-        let h' = Hash.quotient h in
-        get_exn_ k ~h:h' (A.get a i)
+    | N (leaf, a) ->
+        if Hash.is_0 h then get_exn_list_ k leaf
+        else
+          let i = Hash.rem h in
+          let h' = Hash.quotient h in
+          get_exn_ k ~h:h' (A.get a i)
 
   let get_exn k m = get_exn_ k ~h:(hash_ k) m
 
@@ -173,15 +203,24 @@ module Make(Key : KEY)
         if h=h'
         then L (h, add_list_ k v ~h l)
         else (* split into N *)
-          let a = A.create E in
-          (* put leaf in the right bucket *)
-          let i = Hash.rem h' in
-          let h'' = Hash.quotient h' in
-          let a = A.set a i (L (h'', l)) in
+          let a = empty_arr_ in
+          let a, leaf =
+            if Hash.is_0 h' then a, l
+            else
+              (* put leaf in the right bucket *)
+              let i = Hash.rem h' in
+              let h'' = Hash.quotient h' in
+              A.set a i (L (h'', l)), Nil
+          in
           (* then add new node *)
-          let a = add_to_array_ k v ~h a in
-          N a
-    | N a -> N (add_to_array_ k v ~h a)
+          let a, leaf =
+            if Hash.is_0 h then a, add_list_ k v ~h leaf
+            else add_to_array_ k v ~h a, leaf
+          in
+          N (leaf, a)
+    | N (leaf, a) ->
+        if Hash.is_0 h then N (add_list_ k v ~h leaf, a)
+        else N (leaf, add_to_array_ k v ~h a)
 
   (* [left] list nodes already visited *)
   and add_list_ k v ~h l = match l with
@@ -208,6 +247,10 @@ module Make(Key : KEY)
       true
     with LocalExit -> false
 
+  let is_empty_list_ = function
+    | Nil -> true
+    | Cons _ -> false
+
   let rec remove_list_ k l = match l with
     | Nil -> Nil
     | Cons (k', v', tail) ->
@@ -218,17 +261,20 @@ module Make(Key : KEY)
   let rec remove_rec_ k ~h m = match m with
     | E -> E
     | L (h, l) ->
-        begin match remove_list_ k l with
-        | Nil -> E
-        | Cons _ as res -> L (h, res)
-        end
-    | N a ->
-        let i = Hash.rem h in
-        let h' = Hash.quotient h in
-        let a' = A.set a i (remove_rec_ k ~h:h' (A.get a i)) in
-        if is_empty_arr_ a'
+        let l = remove_list_ k l in
+        if is_empty_list_ l then E else L (h, l)
+    | N (leaf, a) ->
+        let leaf, a =
+          if Hash.is_0 h
+            then remove_list_ k leaf, a
+            else
+              let i = Hash.rem h in
+              let h' = Hash.quotient h in
+              leaf, A.set a i (remove_rec_ k ~h:h' (A.get a i))
+        in
+        if is_empty_list_ leaf && is_empty_arr_ a
           then E
-          else N a'
+          else N (leaf, a)
 
   let remove k m = remove_rec_ k ~h:(hash_ k) m
 
@@ -236,7 +282,7 @@ module Make(Key : KEY)
     let rec aux = function
       | E -> ()
       | L (_,l) -> aux_list l
-      | N a -> A.iter aux a
+      | N (l,a) -> aux_list l; A.iter aux a
     and aux_list = function
       | Nil -> ()
       | Cons (k, v, tl) -> f k v; aux_list tl
@@ -247,7 +293,7 @@ module Make(Key : KEY)
     let rec aux acc t = match t with
       | E -> acc
       | L (_,l) -> aux_list acc l
-      | N a -> A.fold aux acc a
+      | N (l,a) -> let acc = aux_list acc l in A.fold aux acc a
     and aux_list acc l = match l with
       | Nil -> acc
       | Cons (k, v, tl) -> let acc = f acc k v in aux_list acc tl
@@ -275,7 +321,7 @@ module Make(Key : KEY)
   let rec as_tree m () = match m with
     | E -> `Nil
     | L (h,l) -> `Node (`L ((h:>int), list_as_tree_ l), [])
-    | N a -> `Node (`N, array_as_tree_ a)
+    | N (l,a) -> `Node (`N, as_tree (L (Hash.zero, l)) :: array_as_tree_ a)
   and list_as_tree_ l = match l with
     | Nil -> []
     | Cons (k, v, tail) -> (k,v) :: list_as_tree_ tail
diff --git a/src/data/CCHashTrie.mli b/src/data/CCHashTrie.mli
index 0082cd07..eb621e72 100644
--- a/src/data/CCHashTrie.mli
+++ b/src/data/CCHashTrie.mli
@@ -19,7 +19,7 @@ type 'a ktree = unit -> [`Nil | `Node of 'a * 'a ktree list]
 
 (** {2 Fixed-Size Arrays} *)
 module type FIXED_ARRAY = sig
-  type 'a t
+  type +'a t
   val create : 'a -> 'a t
   val length_log : int
   val length : int  (* 2 power length_log *)