feat(drup-check): functorize over atoms

src/checker/drup_check.ml

@@ -0,0 +1,211 @@
+
+module SDrup = Sidekick_drup
+
+module Atom : sig
+  include SDrup.ATOM with type t = private int
+
+  val of_int : int -> t
+end = struct
+  type t = int
+  type atom = t
+  let hash = CCHash.int
+  let equal : t -> t -> bool = (=)
+  let compare : t -> t -> int = compare
+  let[@inline] neg x = x lxor 1
+  let[@inline] of_int x =
+    let v = abs x lsl 1 in
+    if x < 0 then neg v else v
+  let[@inline] sign x = (x land 1) = 0
+  let[@inline] to_int x = (if sign x then 1 else -1) * (x lsr 1)
+  let pp out x =
+    Fmt.fprintf out "%s%d" (if sign x then "+" else "-") (x lsr 1)
+  let[@inline] of_int_unsafe i = i
+  let dummy = 0
+  module Assign = struct
+    type t = Bitvec.t
+    let create = Bitvec.create
+    let ensure_size = Bitvec.ensure_size
+    let is_true = Bitvec.get
+    let[@inline] is_false self (a:atom) : bool =
+      is_true self (neg a)
+    let[@inline] is_unassigned self a =
+      not (is_true self a) && not (is_false self a)
+    let set = Bitvec.set
+  end
+  module Map = struct
+    type 'a t = 'a Vec.t
+    let create () = Vec.create ()
+    let[@inline] ensure_has (self:_ t) a mk : unit =
+      (* size: 2+atom, because: 1+atom makes atom valid, and if it's positive,
+         2+atom is (¬atom)+1 *)
+      Vec.ensure_size_with self mk (2+(a:atom:>int))
+    let get = Vec.get
+    let set = Vec.set
+  end
+  module Stack = struct
+    include VecI32
+    let create()=create()
+  end
+end
+
+include SDrup.Make(Atom)
+
+(** A DRUP trace, as a series of operations *)
+module Trace : sig
+  type t
+
+  val create : Clause.store -> t
+  val cstore : t -> Clause.store
+
+  val add_clause : t -> clause -> unit
+  val add_input_clause : t -> clause -> unit
+  val del_clause : t -> clause -> unit
+
+  (** Operator on the set of clauses *)
+  type op =
+    | Input of clause
+    | Redundant of clause
+    | Delete of clause
+
+  val iteri : t -> f:(int -> op -> unit) -> unit
+  val ops : t -> op Iter.t
+  val size : t -> int
+  val get : t -> int -> op
+
+  val pp_op : op Fmt.printer
+
+  val dump : out_channel -> t -> unit
+end = struct
+  type op =
+    | Input of clause
+    | Redundant of clause
+    | Delete of clause
+
+  type t = {
+    cstore: Clause.store;
+    ops: op Vec.t;
+  }
+
+  let create cstore : t =
+    { cstore; ops=Vec.create() }
+
+  let cstore self = self.cstore
+  let add_clause self c = Vec.push self.ops (Redundant c)
+  let add_input_clause self c = Vec.push self.ops (Input c)
+  let del_clause self c = Vec.push self.ops (Delete c)
+  let get self i = Vec.get self.ops i
+  let size self = Vec.size self.ops
+  let ops self = Vec.to_seq self.ops
+  let iteri self ~f = Vec.iteri f self.ops
+
+  let pp_op out = function
+    | Input c -> Fmt.fprintf out "(@[Input %a@])" Clause.pp c
+    | Redundant c -> Fmt.fprintf out "(@[Redundant %a@])" Clause.pp c
+    | Delete c -> Fmt.fprintf out "(@[Delete %a@])" Clause.pp c
+
+  let dump oc self : unit =
+    let fpf = Printf.fprintf in
+    let pp_c out c = Clause.iter c ~f:(fun a -> fpf oc "%d " (a:atom:>int)); in
+    Vec.iter
+      (function
+        | Input c -> fpf oc "i %a0\n" pp_c c;
+        | Redundant c -> fpf oc "%a0\n" pp_c c;
+        | Delete c -> fpf oc "d %a0\n" pp_c c;
+      )
+      self.ops
+end
+
+(** Forward checking.
+
+    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
+    ]
+
+  val pp_error : Trace.t -> error Fmt.printer
+
+  (** [check tr] checks the trace and returns [Ok ()] in case of
+      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)
+
+  type t = {
+    checker: Checker.t;
+    errors: VecI32.t;
+  }
+  let create cstore : t = {
+    checker=Checker.create cstore;
+    errors=VecI32.create();
+  }
+
+  (* check event, return [true] if it's valid *)
+  let check_op (self:t) i (op:Trace.op) : bool =
+    Profile.with_ "check-op" @@ fun() ->
+    Log.debugf 20 (fun k->k"(@[check-op :idx %d@ :op %a@])" i Trace.pp_op op);
+
+    begin match op with
+      | Trace.Input c ->
+        Checker.add_clause self.checker c;
+        true
+
+      | Trace.Redundant c ->
+
+        let ok = Checker.is_valid_drup self.checker c in
+        Checker.add_clause self.checker c; (* now add clause *)
+        ok
+
+      | Trace.Delete c ->
+        Checker.del_clause self.checker c;
+        true
+
+    end
+
+  type error =
+    [ `Bad_steps of VecI32.t
+    | `No_empty_clause
+    ]
+
+  let pp_error trace out = function
+    | `No_empty_clause -> Fmt.string out "no empty clause found"
+    | `Bad_steps bad ->
+      let n0 = VecI32.get bad 0 in
+      Fmt.fprintf out
+        "@[<v>checking failed on %d ops.@ @[<2>First failure is op[%d]:@ %a@]@]"
+        (VecI32.size bad) n0
+        Trace.pp_op (Trace.get trace n0)
+
+  let check trace : _ result =
+    let self = create (Trace.cstore trace) in
+
+    (* check each event in turn *)
+    let has_false = ref false in
+    Trace.iteri trace
+      ~f:(fun i op ->
+          let ok = check_op self i op in
+          if ok then (
+            Log.debugf 50
+              (fun k->k"(@[check.step.ok@ :idx %d@ :op %a@])" i Trace.pp_op op);
+
+            (* check if op adds the empty clause *)
+            begin match op with
+              | (Trace.Redundant c | Trace.Input c) when Clause.size c = 0 ->
+                has_false := true
+              | _ -> ()
+            end;
+          ) else (
+            Log.debugf 10
+              (fun k->k"(@[check.step.fail@ :idx %d@ :op %a@])" i Trace.pp_op op);
+            VecI32.push self.errors i
+          ));
+
+    Log.debugf 10 (fun k->k"found %d errors" (VecI32.size self.errors));
+    if not !has_false then Error `No_empty_clause
+    else if VecI32.size self.errors > 0 then Error (`Bad_steps self.errors)
+    else Ok ()
+end
+
+

src/checker/main.ml

@@ -1,16 +1,15 @@
 
 module BL = Sidekick_bin_lib
-module SDrup = Sidekick_drup
 
-let clause_of_int_l store atoms : SDrup.clause =
+let clause_of_int_l store atoms : Drup_check.clause =
   atoms
-  |> CCList.map SDrup.Atom.of_int
+  |> CCList.map Drup_check.Atom.of_int
-  |> SDrup.Clause.of_list store
+  |> Drup_check.Clause.of_list store
 
 let check ?pb proof : bool =
   Profile.with_ "check" @@ fun() ->
-  let cstore = SDrup.Clause.create() in
+  let cstore = Drup_check.Clause.create() in
-  let trace = SDrup.Trace.create cstore in
+  let trace = Drup_check.Trace.create cstore in
 
   (* add problem to trace, if provided *)
   begin match pb with
@@ -23,7 +22,7 @@ let check ?pb proof : bool =
            BL.Dimacs_parser.iter parser_
              (fun atoms ->
                 let c = clause_of_int_l cstore atoms in
-                SDrup.Trace.add_input_clause trace c))
+                Drup_check.Trace.add_input_clause trace c))
     | Some f ->
       (* TODO: handle .cnf.gz *)
       Error.errorf "unknown problem file extension '%s'" (Filename.extension f)
@@ -40,21 +39,21 @@ let check ?pb proof : bool =
              (function
                | BL.Drup_parser.Add c ->
                  let c = clause_of_int_l cstore c in
-                 SDrup.Trace.add_clause trace c
+                 Drup_check.Trace.add_clause trace c
                | BL.Drup_parser.Delete c ->
                  let c = clause_of_int_l cstore c in
-                 SDrup.Trace.del_clause trace c))
+                 Drup_check.Trace.del_clause trace c))
     | f ->
       (* TODO: handle .drup.gz *)
       Error.errorf "unknown proof file extension '%s'" (Filename.extension f)
   end;
 
   (* check proof *)
-  Log.debugf 1 (fun k->k"checking proof (%d steps)" (SDrup.Trace.size trace));
+  Log.debugf 1 (fun k->k"checking proof (%d steps)" (Drup_check.Trace.size trace));
-  begin match SDrup.Fwd_check.check trace with
+  begin match Drup_check.Fwd_check.check trace with
     | Ok () -> true
     | Error err ->
-      Format.eprintf "%a@." (SDrup.Fwd_check.pp_error trace) err;
+      Format.eprintf "%a@." (Drup_check.Fwd_check.pp_error trace) err;
       false
   end
 

src/drup/sidekick_drup.ml

@@ -1,11 +1,16 @@
 
+(** DRUP trace checker.
+
+    This module provides a checker for DRUP traces, including step-by-step
+    checking for traces that interleave DRUP steps with other kinds of steps.
+*)
+
 module Fmt = CCFormat
 module VecI32 = VecI32
 
-module Atom : sig
+(** Signature for boolean atoms *)
-  type t = private int
+module type ATOM = sig
-  val of_int : int -> t
+  type t
-  val to_int : t -> int
   val equal : t -> t -> bool
   val compare : t -> t -> int
   val hash : t -> int
@@ -14,25 +19,83 @@ module Atom : sig
   val pp : t Fmt.printer
 
   val dummy : t
-  val of_int_unsafe : int -> t
+
-  module Map : CCMap.S with type key = t
+  type atom = t
-end = struct
+
-  type t = int
+  module Assign : sig
-  let hash = CCHash.int
+    type t
-  let equal : t -> t -> bool = (=)
+    val create : unit -> t
-  let compare : t -> t -> int = compare
+
-  let neg x = x lxor 1
+    val ensure_size : t -> atom -> unit
-  let of_int x =
+    val set : t -> atom -> bool -> unit
-    let v = abs x lsl 1 in
+    val is_true : t -> atom -> bool
-    if x < 0 then neg v else v
+    val is_false : t -> atom -> bool
-  let sign x = (x land 1) = 0
+    val is_unassigned : t -> atom -> bool
-  let to_int x = (if sign x then 1 else -1) * (x lsr 1)
-  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
+
+  module Map : sig
+    type 'a t
+    val create : unit -> 'a t
+    val ensure_has : 'a t -> atom -> (unit -> 'a) -> unit
+    val get : 'a t -> atom -> 'a
+  end
+
+  module Stack : sig
+    type t
+    val create : unit -> t
+    val get : t -> int -> atom
+    val set : t -> int -> atom -> unit
+    val push : t -> atom -> unit
+    val size : t -> int
+    val shrink : t -> int -> unit
+    val to_iter : t -> atom Iter.t
+  end
+end
+
+(* TODO: resolution proof construction, optionally *)
+
+(* TODO: backward checking + pruning of traces *)
+
+(** An instance of the checker *)
+module type S = sig
+  type atom
+
+  module Clause : sig
+
+    type store
+    val create : unit -> store
+
+    type t
+
+    val size : t -> int
+
+    val get : t -> int -> atom
+
+    val iter : f:(atom -> unit) -> t -> unit
+
+    val pp : t Fmt.printer
+
+    val of_list : store -> atom list -> t
+  end
+  type clause = Clause.t
+
+  module Checker : sig
+    type t
+
+    val create : Clause.store -> t
+
+    val add_clause : t -> Clause.t -> unit
+
+    val is_valid_drup : t -> Clause.t -> bool
+
+    val del_clause : t -> Clause.t -> unit
+  end
+end
+
+module[@inline] Make(A : ATOM)
+  : S with type atom = A.t
+= struct
+  module Atom = A
   type atom = Atom.t
 
   (** Boolean clauses *)
@@ -41,6 +104,7 @@ module Clause : sig
     val create : unit -> store
     type t
     val size : t -> int
+    val id : t -> int
     val get : t -> int -> atom
     val iter : f:(atom -> unit) -> t -> unit
     val watches: t -> atom * atom
@@ -50,11 +114,6 @@ module Clause : sig
     module Set : CCSet.S with type elt = t
     module Tbl : CCHashtbl.S with type key = t
   end = struct
-  module I_arr_tbl = CCHashtbl.Make(struct
-      type t = atom array
-      let equal = CCEqual.(array Atom.equal)
-      let hash = CCHash.(array Atom.hash)
-    end)
     type t = {
       id: int;
       atoms: atom array;
@@ -65,6 +124,7 @@ end = struct
     }
     let create(): store =
       { n=0; }
+    let[@inline] id self = self.id
     let[@inline] size self = Array.length self.atoms
     let[@inline] get self i = Array.get self.atoms i
     let[@inline] watches self = self.watches
@@ -97,131 +157,54 @@ end = struct
   end
   type clause = Clause.t
 
-(** A DRUP trace, as a series of operations *)
+  (** Forward proof checker.
-module Trace : sig
-  type t
-
-  val create : Clause.store -> t
-  val cstore : t -> Clause.store
-
-  val add_clause : t -> clause -> unit
-  val add_input_clause : t -> clause -> unit
-  val del_clause : t -> clause -> unit
-
-  (** Operator on the set of clauses *)
-  type op =
-    | Input of clause
-    | Redundant of clause
-    | Delete of clause
-
-  val iteri : t -> f:(int -> op -> unit) -> unit
-  val ops : t -> op Iter.t
-  val size : t -> int
-  val get : t -> int -> op
-
-  val pp_op : op Fmt.printer
-
-  val dump : out_channel -> t -> unit
-end = struct
-  type op =
-    | Input of clause
-    | Redundant of clause
-    | Delete of clause
-
-  type t = {
-    cstore: Clause.store;
-    ops: op Vec.t;
-  }
-
-  let create cstore : t =
-    { cstore; ops=Vec.create() }
-
-  let cstore self = self.cstore
-  let add_clause self c = Vec.push self.ops (Redundant c)
-  let add_input_clause self c = Vec.push self.ops (Input c)
-  let del_clause self c = Vec.push self.ops (Delete c)
-  let get self i = Vec.get self.ops i
-  let size self = Vec.size self.ops
-  let ops self = Vec.to_seq self.ops
-  let iteri self ~f = Vec.iteri f self.ops
-
-  let pp_op out = function
-    | Input c -> Fmt.fprintf out "(@[Input %a@])" Clause.pp c
-    | Redundant c -> Fmt.fprintf out "(@[Redundant %a@])" Clause.pp c
-    | Delete c -> Fmt.fprintf out "(@[Delete %a@])" Clause.pp c
-
-  let dump oc self : unit =
-    let fpf = Printf.fprintf in
-    let pp_c out c = Clause.iter c ~f:(fun a -> fpf oc "%d " (Atom.to_int a)); in
-    Vec.iter
-      (function
-        | Input c -> fpf oc "i %a0\n" pp_c c;
-        | Redundant c -> fpf oc "%a0\n" pp_c c;
-        | Delete c -> fpf oc "d %a0\n" pp_c c;
-      )
-      self.ops
-end
-
-(** Forward checking.
 
       Each event is checked by reverse-unit propagation on previous events. *)
-module Fwd_check : sig
+  module Checker : sig
-  type error =
+    type t
-    [ `Bad_steps of VecI32.t
+    val create : Clause.store -> t
-    | `No_empty_clause
+    val add_clause : t -> Clause.t -> unit
-    ]
+    val is_valid_drup : t -> Clause.t -> bool
-
+    val del_clause : t -> Clause.t -> unit
-  val pp_error : Trace.t -> error Fmt.printer
-
-  (** [check tr] checks the trace and returns [Ok ()] in case of
-      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)
-
     type t = {
       cstore: Clause.store;
-    assign: Bitvec.t; (* atom -> is_true(atom) *)
+      assign: Atom.Assign.t; (* atom -> is_true(atom) *)
-    trail: VecI32.t; (* current assignment *)
+      trail: Atom.Stack.t; (* current assignment *)
       mutable trail_ptr : int; (* offset in trail for propagation *)
       active_clauses: unit Clause.Tbl.t;
-    watches: Clause.t Vec.t Vec.t; (* atom -> clauses it watches *)
+      watches: Clause.t Vec.t Atom.Map.t; (* atom -> clauses it watches *)
-    errors: VecI32.t;
     }
 
     let create cstore : t =
-    { trail=VecI32.create();
+      { trail=Atom.Stack.create();
         trail_ptr = 0;
         cstore;
         active_clauses=Clause.Tbl.create 32;
-      assign=Bitvec.create();
+        assign=Atom.Assign.create();
-      watches=Vec.create();
+        watches=Atom.Map.create();
-      errors=VecI32.create();
       }
 
     (* ensure data structures are big enough to handle [a] *)
     let ensure_atom_ self (a:atom) =
-    Bitvec.ensure_size self.assign (a:atom:>int);
+      Atom.Assign.ensure_size self.assign a;
       (* size: 2+atom, because: 1+atom makes atom valid, and if it's positive,
          2+atom is (¬atom)+1 *)
-    Vec.ensure_size_with self.watches Vec.create (2+(a:atom:>int));
+      Atom.Map.ensure_has self.watches a (fun _ -> Vec.create ());
       ()
 
     let[@inline] is_true self (a:atom) : bool =
-    Bitvec.get self.assign (a:atom:>int)
+      Atom.Assign.is_true self.assign a
-
     let[@inline] is_false self (a:atom) : bool =
-    is_true self (Atom.neg a)
+      Atom.Assign.is_false self.assign a
-
+    let[@inline] is_unassigned self a =
-  let is_unassigned self a =
+      Atom.Assign.is_unassigned self.assign a
-    not (is_true self a) && not (is_false self a)
 
     let add_watch_ self (a:atom) (c:clause) =
-    Vec.push (Vec.get self.watches (a:atom:>int)) c
+      Vec.push (Atom.Map.get self.watches a) c
 
     let remove_watch_ self (a:atom) idx =
-    let v = Vec.get self.watches (a:atom:>int) in
+      let v = Atom.Map.get self.watches a in
       Vec.fast_remove v idx
 
     exception Conflict
@@ -231,18 +214,17 @@ end = struct
       raise Conflict
 
     (* set atom to true *)
-  let set_atom_true (self:t) (a:atom) : unit =
+    let[@inline] set_atom_true (self:t) (a:atom) : unit =
       if is_true self a then ()
       else if is_false self a then raise_conflict_ self a
       else (
-      Bitvec.set self.assign (a:atom:>int) true;
+        Atom.Assign.set self.assign a true;
-      VecI32.push self.trail (a:atom:>int)
+        Atom.Stack.push self.trail a
       )
 
     (* print the trail *)
     let pp_trail_ out self =
-    let pp_a out i = Atom.pp out (Atom.of_int_unsafe i) in
+      Fmt.fprintf out "(@[%a@])" (Fmt.iter Atom.pp) (Atom.Stack.to_iter self.trail)
-    Fmt.fprintf out "(@[%a@])" (Fmt.iter pp_a) (VecI32.to_iter self.trail)
 
     exception Found_watch of atom
     exception Is_sat
@@ -310,7 +292,7 @@ end = struct
       | Is_sat -> Keep
 
     let propagate_atom_ self (a:atom) : unit =
-    let v = Vec.get self.watches (a:atom:>int) in
+      let v = Atom.Map.get self.watches a in
       let i = ref 0 in
       while !i < Vec.size v do
         match propagate_in_clause_ self a (Vec.get v !i) with
@@ -323,8 +305,8 @@ end = struct
        @raise Conflict if a clause is false *)
     let bcp_fixpoint_ (self:t) : unit =
       Profile.with_ "bcp-fixpoint" @@ fun() ->
-    while self.trail_ptr < VecI32.size self.trail do
+      while self.trail_ptr < Atom.Stack.size self.trail do
-      let a = Atom.of_int_unsafe (VecI32.get self.trail self.trail_ptr) in
+        let a = Atom.Stack.get self.trail self.trail_ptr in
         Log.debugf 50 (fun k->k"(@[bcp@ :atom %a@])" Atom.pp a);
         self.trail_ptr <- 1 + self.trail_ptr;
         propagate_atom_ self a;
@@ -332,31 +314,26 @@ end = struct
 
     (* calls [f] and then restore trail to what it was *)
     let with_restore_trail_ self f =
-    let trail_size0 = VecI32.size self.trail in
+      let trail_size0 = Atom.Stack.size self.trail in
       let ptr0 = self.trail_ptr in
 
       let restore () =
         (* unassign new literals *)
-      for i=trail_size0 to VecI32.size self.trail - 1 do
+        for i=trail_size0 to Atom.Stack.size self.trail - 1 do
-        let a = Atom.of_int_unsafe (VecI32.get self.trail i) in
+          let a = Atom.Stack.get self.trail i in
           assert (is_true self a);
-        Bitvec.set self.assign (a:atom:>int) false;
+          Atom.Assign.set self.assign a false;
         done;
 
         (* remove literals from trail *)
-      VecI32.shrink self.trail trail_size0;
+        Atom.Stack.shrink self.trail trail_size0;
         self.trail_ptr <- ptr0
       in
 
       CCFun.finally ~h:restore ~f
 
-  (* check event, return [true] if it's valid *)
-  let check_op (self:t) i (op:Trace.op) : bool =
-    Profile.with_ "check-op" @@ fun() ->
-    Log.debugf 20 (fun k->k"(@[check-op :idx %d@ :op %a@])" i Trace.pp_op op);
-
     (* add clause to the state *)
-    let add_c_ (c:Clause.t) =
+    let add_clause (self:t) (c:Clause.t) =
       Log.debugf 50 (fun k->k"(@[add-clause@ %a@])" Clause.pp c);
       Clause.iter c ~f:(ensure_atom_ self);
       Clause.Tbl.add self.active_clauses c ();
@@ -387,18 +364,10 @@ end = struct
           )
       end;
       ()
-    in
-
-    match op with
-    | Trace.Input c ->
-      add_c_ c;
-      true
-
-    | Trace.Redundant c ->
 
+    let is_valid_drup (self:t) (c:Clause.t) : bool =
       (* negate [c], pushing each atom on trail, and see if we get [Conflict]
          by pure propagation *)
-      let ok =
       try
         with_restore_trail_ self @@ fun () ->
         Clause.iter c
@@ -422,59 +391,10 @@ end = struct
         false
       with Conflict ->
         true
-      in
 
-      (* now add clause *)
+    let del_clause (_self:t) (_c:Clause.t) : unit =
-      add_c_ c;
+      () (* TODO *)
-      ok
-
-    | Trace.Delete _c ->
-      true (* TODO: actually remove the clause *)
-
-  type error =
-    [ `Bad_steps of VecI32.t
-    | `No_empty_clause
-    ]
-
-  let pp_error trace out = function
-    | `No_empty_clause -> Fmt.string out "no empty clause found"
-    | `Bad_steps bad ->
-      let n0 = VecI32.get bad 0 in
-      Fmt.fprintf out
-        "@[<v>checking failed on %d ops.@ @[<2>First failure is op[%d]:@ %a@]@]"
-        (VecI32.size bad) n0
-        Trace.pp_op (Trace.get trace n0)
-
-  let check trace : _ result =
-    let self = create (Trace.cstore trace) in
-
-    (* check each event in turn *)
-    let has_false = ref false in
-    Trace.iteri trace
-      ~f:(fun i op ->
-          let ok = check_op self i op in
-          if ok then (
-            Log.debugf 50
-              (fun k->k"(@[check.step.ok@ :idx %d@ :op %a@])" i Trace.pp_op op);
-
-            (* check if op adds the empty clause *)
-            begin match op with
-              | (Trace.Redundant c | Trace.Input c) when Clause.size c = 0 ->
-                has_false := true
-              | _ -> ()
-            end;
-          ) else (
-            Log.debugf 10
-              (fun k->k"(@[check.step.fail@ :idx %d@ :op %a@])" i Trace.pp_op op);
-            Log.debugf 50 (fun k->k"(@[trail: %a@])" pp_trail_ self);
-            VecI32.push self.errors i
-          ));
-
-    Log.debugf 10 (fun k->k"found %d errors" (VecI32.size self.errors));
-    if not !has_false then Error `No_empty_clause
-    else if VecI32.size self.errors > 0 then Error (`Bad_steps self.errors)
-    else Ok ()
   end
 
+end
 
-(* TODO: backward checking + pruning of traces *)