diff --git a/src/checker/dune b/src/checker/dune
index fab0d26a..895db754 100644
--- a/src/checker/dune
+++ b/src/checker/dune
@@ -3,6 +3,6 @@
   (name main)
   (public_name sidekick-checker)
   (package sidekick-bin)
-  (libraries containers sidekick-bin.lib
+  (libraries containers sidekick-bin.lib mtime mtime.clock.os
              sidekick.util sidekick.tef sidekick.drup)
   (flags :standard -warn-error -a+8 -open Sidekick_util))
diff --git a/src/checker/main.ml b/src/checker/main.ml
index ad7d5660..9fc84cab 100644
--- a/src/checker/main.ml
+++ b/src/checker/main.ml
@@ -68,14 +68,18 @@ let () =
 
   Arg.parse opts (fun f -> files := f :: !files) "checker [opt]* [file]+";
 
-  begin match List.rev !files with
+  let ok =
+    match List.rev !files with
     | [pb; proof] ->
       Log.debugf 1 (fun k->k"checker: problem `%s`, proof `%s`" pb proof);
-      let ok = check ~pb proof in
-      if not ok then exit 1
+      check ~pb proof
     | [proof] ->
       Log.debugf 1 (fun k->k"checker: proof `%s`" proof);
-      let ok = check ?pb:None proof in
-      if not ok then exit 1
-    | _ -> failwith "expected <problem>? <proof>"
-  end
+      check ?pb:None proof
+    | _ -> Error.errorf "expected <problem>? <proof>"
+  in
+
+  let t2 = Mtime_clock.elapsed () |> Mtime.Span.to_s in
+  Format.printf "c %s@." (if ok then "OK" else "FAIL");
+  Format.printf "c elapsed time: %.3fs@." t2;
+  if not ok then exit 1
diff --git a/src/drup/sidekick_drup.ml b/src/drup/sidekick_drup.ml
index eb2bbaff..baebfcaa 100644
--- a/src/drup/sidekick_drup.ml
+++ b/src/drup/sidekick_drup.ml
@@ -13,6 +13,7 @@ module Atom : sig
   val sign : t -> bool
   val pp : t Fmt.printer
 
+  val dummy : t
   val of_int_unsafe : int -> t
   module Map : CCMap.S with type key = t
 end = struct
@@ -29,6 +30,7 @@ end = struct
   let pp out x =
     Fmt.fprintf out "%s%d" (if sign x then "+" else "-") (x lsr 1)
   let of_int_unsafe i = i
+  let dummy = 0
   module Map = Util.Int_map
 end
 type atom = Atom.t
@@ -41,7 +43,7 @@ module Clause : sig
   val size : t -> int
   val get : t -> int -> atom
   val iter : f:(atom -> unit) -> t -> unit
-  val watches: t -> (atom * atom) option
+  val watches: t -> atom * atom
   val set_watches : t -> atom * atom -> unit
   val pp : t Fmt.printer
   val of_list : store -> atom list -> t
@@ -56,7 +58,7 @@ end = struct
   type t = {
     id: int;
     atoms: atom array;
-    mutable watches: (atom * atom) option;
+    mutable watches: atom * atom;
   }
   type store = {
     mutable n: int;
@@ -66,12 +68,12 @@ end = struct
   let size self = Array.length self.atoms
   let get self i = Array.get self.atoms i
   let watches self = self.watches
-  let set_watches self w = self.watches <- Some w
+  let set_watches self w = self.watches <- w
   let iter ~f self = Array.iter f self.atoms
   let pp out (self:t) =
     let pp_watches out = function
-      | None -> ()
-      | Some (p,q) -> Fmt.fprintf out "@ :watches (%a,%a)" Atom.pp p Atom.pp q in
+      | (p,q) when p=Atom.dummy || q=Atom.dummy -> ()
+      | (p,q) -> Fmt.fprintf out "@ :watches (%a,%a)" Atom.pp p Atom.pp q in
     Fmt.fprintf out "(@[cl[%d]@ %a%a])"
       self.id (Fmt.Dump.array Atom.pp) self.atoms pp_watches self.watches
   let of_list self atoms : t =
@@ -79,7 +81,7 @@ end = struct
     let atoms = List.sort_uniq Atom.compare atoms |> Array.of_list in
     let id = self.n in
     self.n <- 1 + self.n;
-    let c = {atoms; id; watches=None} in
+    let c = {atoms; id; watches=Atom.dummy, Atom.dummy} in
     c
   module As_key = struct
     type nonrec t=t
@@ -161,7 +163,6 @@ end
 
     Each event is checked by reverse-unit propagation on previous events. *)
 module Fwd_check : sig
-
   type error =
     [ `Bad_steps of VecI32.t
     | `No_empty_clause
@@ -173,7 +174,6 @@ module Fwd_check : sig
       success. In case of error it returns [Error idxs] where [idxs] are the
       indexes in the trace of the steps that failed. *)
   val check : Trace.t -> (unit, error) result
-
 end = struct
   module ISet = CCSet.Make(CCInt)
 
@@ -183,7 +183,7 @@ end = struct
     trail: VecI32.t; (* current assignment *)
     mutable trail_ptr : int; (* offset in trail for propagation *)
     active_clauses: unit Clause.Tbl.t;
-    watches: Clause.Set.t Vec.t; (* atom -> clauses it watches *)
+    watches: Clause.t Vec.t Vec.t; (* atom -> clauses it watches *)
     errors: VecI32.t;
   }
 
@@ -202,7 +202,7 @@ end = struct
     Bitvec.ensure_size self.assign (a:atom:>int);
     (* size: 2+atom, because: 1+atom makes atom valid, and if it's positive,
        2+atom is (¬atom)+1 *)
-    Vec.ensure_size self.watches Clause.Set.empty (2+(a:atom:>int));
+    Vec.ensure_size_with self.watches Vec.create (2+(a:atom:>int));
     ()
 
   let[@inline] is_true self (a:atom) : bool =
@@ -215,12 +215,11 @@ end = struct
     not (is_true self a) && not (is_false self a)
 
   let add_watch_ self (a:atom) (c:clause) =
-    let set = Vec.get self.watches (a:atom:>int) in
-    Vec.set self.watches (a:atom:>int) (Clause.Set.add c set)
+    Vec.push (Vec.get self.watches (a:atom:>int)) c
 
-  let remove_watch_ self (a:atom) (c:clause) =
-    let set = Vec.get self.watches (a:atom:>int) in
-    Vec.set self.watches (a:atom:>int) (Clause.Set.remove c set)
+  let remove_watch_ self (a:atom) idx =
+    let v = Vec.get self.watches (a:atom:>int) in
+    Vec.fast_remove v idx
 
   exception Conflict
 
@@ -251,19 +250,19 @@ end = struct
     try Clause.iter c ~f:(fun a -> if not (is_false self a) then raise Exit); true
     with Exit -> false
 
+  type propagation_res =
+    | Keep
+    | Remove
+
   (* do boolean propagation in [c], which is watched by the true literal [a] *)
-  let propagate_in_clause_ (self:t) (a:atom) (c:clause) : unit =
+  let propagate_in_clause_ (self:t) (a:atom) (c:clause) : propagation_res =
     assert (is_true self a);
-    let a1, a2 =
-      match Clause.watches c with
-      | None -> assert false
-      | Some tup -> tup
-    in
+    let a1, a2 = Clause.watches c in
     let na = Atom.neg a in
     (* [q] is the other literal in [c] such that [¬q] watches [c]. *)
     let q = if Atom.equal a1 na then a2 else (assert(a2==na); a1) in
     try
-      if is_true self q then () (* clause is satisfied *)
+      if is_true self q then Keep (* clause is satisfied *)
       else (
         let n_unassigned = ref 0 in
         let unassigned_a = ref a in (* an unassigned atom, if [!n_unassigned > 0] *)
@@ -294,21 +293,28 @@ end = struct
           let p = !unassigned_a in
           Log.debugf 30 (fun k->k"(@[propagate@ :atom %a@ :reason %a@])" Atom.pp p Clause.pp c);
           set_atom_true self p;
+          Keep
         ) else (
           (* at least 2 unassigned, just update the watch literal to [¬p] *)
           let p = !unassigned_a in
           assert (p <> q);
           Clause.set_watches c (q, p);
-          remove_watch_ self a c;
           add_watch_ self (Atom.neg p) c;
+          Remove
         );
       )
     with
-    | Is_sat -> ()
+    | Is_sat -> Keep
 
   let propagate_atom_ self (a:atom) : unit =
-    let set = Vec.get self.watches (a:atom:>int) in
-    Clause.Set.iter (propagate_in_clause_ self a) set
+    let v = Vec.get self.watches (a:atom:>int) in
+    let i = ref 0 in
+    while !i < Vec.size v do
+      match propagate_in_clause_ self a (Vec.get v !i) with
+      | Keep -> incr i;
+      | Remove ->
+        remove_watch_ self a !i
+    done
 
   (* perform boolean propagation in a fixpoint
      @raise Conflict if a clause is false *)
@@ -360,15 +366,21 @@ end = struct
           let c0 = Clause.get c 0 in
           let c1 = Clause.get c 1 in
           assert (c0 <> c1);
-          add_watch_ self (Atom.neg c0) c;
-          add_watch_ self (Atom.neg c1) c;
           Clause.set_watches c (c0,c1);
           (* make sure watches are valid *)
           if is_false self c0 then (
-            propagate_in_clause_ self (Atom.neg c0) c;
+            match propagate_in_clause_ self (Atom.neg c0) c with
+            | Keep -> add_watch_ self (Atom.neg c0) c;
+            | Remove -> ()
+          ) else (
+            add_watch_ self (Atom.neg c0) c
           );
           if is_false self c1 then (
-            propagate_in_clause_ self (Atom.neg c1) c;
+            match propagate_in_clause_ self (Atom.neg c1) c with
+            | Keep -> add_watch_ self (Atom.neg c1) c;
+            | Remove -> ()
+          ) else (
+            add_watch_ self (Atom.neg c1) c
           )
       end;
       ()
diff --git a/src/util/Vec.ml b/src/util/Vec.ml
index b6a54315..3353f6dd 100644
--- a/src/util/Vec.ml
+++ b/src/util/Vec.ml
@@ -48,15 +48,20 @@ let resize_ t x size =
 let ensure_cap_ self x n =
   if n > Array.length self.data then (
     let new_size = max n (2 * Array.length self.data) in
-    resize_ self x new_size
+    resize_ self (x()) new_size
   )
 
-let ensure_size self x n =
-  ensure_cap_ self x n;
+let ensure_size_with self f n =
+  ensure_cap_ self f n;
   if n > self.sz then (
+    for i=self.sz to n-1 do
+      self.data.(i) <- f();
+    done;
     self.sz <- n
   )
 
+let ensure_size self x n = ensure_size_with self (fun() -> x) n
+
 (* grow the array *)
 let[@inline never] grow_to_double_size t x : unit =
   let new_size =
diff --git a/src/util/Vec.mli b/src/util/Vec.mli
index 5c68b6fa..5e0038b3 100644
--- a/src/util/Vec.mli
+++ b/src/util/Vec.mli
@@ -31,6 +31,9 @@ val clear : 'a t -> unit
 val ensure_size : 'a t -> 'a -> int -> unit
 (** ensure size is at least [n] *)
 
+val ensure_size_with : 'a t -> (unit -> 'a) -> int -> unit
+(** ensure size is at least [n] *)
+
 val shrink : 'a t -> int -> unit
 (** [shrink vec sz] resets size of [vec] to [sz].
     Assumes [sz >=0 && sz <= size vec] *)