diff --git a/src/intsolver/dune b/src/intsolver/dune
new file mode 100644
index 00000000..04779850
--- /dev/null
+++ b/src/intsolver/dune
@@ -0,0 +1,6 @@
+(library
+  (name sidekick_intsolver)
+  (public_name sidekick.intsolver)
+  (synopsis "Simple integer solver")
+  (flags :standard -warn-error -a+8 -w -32 -open Sidekick_util)
+  (libraries containers sidekick.core sidekick.arith))
diff --git a/src/intsolver/sidekick_intsolver.ml b/src/intsolver/sidekick_intsolver.ml
new file mode 100644
index 00000000..f5e4fd1d
--- /dev/null
+++ b/src/intsolver/sidekick_intsolver.ml
@@ -0,0 +1,304 @@
+
+module type ARG = sig
+  module Z : Sidekick_arith.INT
+
+  type term
+  type lit
+
+  val pp_term : term Fmt.printer
+  val pp_lit : lit Fmt.printer
+
+  module T_map : CCMap.S with type key = term
+end
+
+module type S = sig
+  module A : ARG
+
+  module Op : sig
+    type t =
+      | Leq
+      | Lt
+      | Eq
+    val pp : t Fmt.printer
+  end
+
+  type t
+
+  val create : unit -> t
+
+  val push_level : t -> unit
+
+  val pop_levels : t -> int -> unit
+
+  val assert_ :
+    t ->
+    (A.Z.t * A.term) list -> Op.t -> A.Z.t ->
+    lit:A.lit ->
+    unit
+
+  val define :
+    t ->
+    A.term ->
+    (A.Z.t * A.term) list ->
+    unit
+
+  module Cert : sig
+    type t
+    val pp : t Fmt.printer
+
+    val lits : t -> A.lit Iter.t
+  end
+
+  module Model : sig
+    type t
+    val pp : t Fmt.printer
+
+    val eval : t -> A.term -> A.Z.t option
+  end
+
+  type result =
+    | Sat of Model.t
+    | Unsat of Cert.t
+
+  val pp_result : result Fmt.printer
+
+  val check : t -> result
+
+  (**/**)
+  val _check_invariants : t -> unit
+  (**/**)
+end
+
+
+
+module Make(A : ARG)
+  : S with module A = A
+= struct
+  module BVec = Backtrack_stack
+  module A = A
+  open A
+
+  module Op = struct
+    type t =
+      | Leq
+      | Lt
+      | Eq
+
+    let pp out = function
+      | Leq -> Fmt.string out "<="
+      | Lt -> Fmt.string out "<"
+      | Eq -> Fmt.string out "="
+  end
+
+  module Linexp = struct
+    type t = Z.t T_map.t
+    let is_empty = T_map.is_empty
+    let empty : t = T_map.empty
+
+    let pp out (self:t) : unit =
+      let pp_pair out (t,z) = Fmt.fprintf out "%a · %a" Z.pp z A.pp_term t in
+      if is_empty self then Fmt.string out "0"
+      else Fmt.fprintf out "(@[+@ %a@])"
+          Fmt.(iter ~sep:(return "@ ") pp_pair) (T_map.to_iter self)
+
+    let iter = T_map.iter
+    let return t : t = T_map.add t Z.one empty
+    let neg self : t = T_map.map Z.neg self
+    let mult n self =
+      if Z.(n = zero) then empty
+      else T_map.map (fun c -> Z.(c * n)) self
+
+    let add (self:t) (c:Z.t) (t:term) : t =
+      let n = Z.(c + T_map.get_or ~default:Z.zero t self) in
+      if Z.(n = zero)
+      then T_map.remove t self
+      else T_map.add t n self
+
+    let merge (self:t) (other:t) : t =
+      T_map.fold
+        (fun t c m -> add m c t)
+        other self
+
+    let of_list l : t =
+      List.fold_left (fun self (c,t) -> add self c t) empty l
+
+    (* map each term to a linexp *)
+    let flat_map f (self:t) : t =
+      T_map.fold
+        (fun t c m ->
+           let t_le = mult c (f t) in
+           merge m t_le
+        )
+        empty self
+  end
+
+  module Cert = struct
+    type t = unit
+    let pp = Fmt.unit
+
+    let lits _ = Iter.empty (* TODO *)
+  end
+
+  module Model = struct
+    type t = {
+      m: Z.t T_map.t;
+    } [@@unboxed]
+
+    let pp out self =
+      let pp_pair out (t,z) = Fmt.fprintf out "(@[%a := %a@])" A.pp_term t Z.pp z in
+      Fmt.fprintf out "(@[model@ %a@])"
+        Fmt.(iter ~sep:(return "@ ") pp_pair) (T_map.to_iter self.m)
+
+    let empty : t = {m=T_map.empty}
+
+    let eval (self:t) t : Z.t option = T_map.get t self.m
+  end
+
+  module Constr = struct
+    type t = {
+      le: Linexp.t;
+      const: Z.t;
+      op: Op.t;
+      lits: lit Bag.t;
+    }
+
+    let pp out self =
+      Fmt.fprintf out "(@[%a@ %a %a@])" Linexp.pp self.le Op.pp self.op Z.pp self.const
+  end
+
+  type t = {
+    defs: (term * Linexp.t) BVec.t;
+    cs: Constr.t BVec.t;
+  }
+
+  let create() : t =
+    { defs=BVec.create();
+      cs=BVec.create(); }
+
+  let push_level self =
+    BVec.push_level self.defs;
+    BVec.push_level self.cs;
+    ()
+
+  let pop_levels self n =
+    BVec.pop_levels self.defs n ~f:(fun _ -> ());
+    BVec.pop_levels self.cs n ~f:(fun _ -> ());
+    ()
+
+  type result =
+    | Sat of Model.t
+    | Unsat of Cert.t
+
+  let pp_result out = function
+    | Sat m -> Fmt.fprintf out "(@[SAT@ %a@])" Model.pp m
+    | Unsat cert -> Fmt.fprintf out "(@[UNSAT@ %a@])" Cert.pp cert
+
+  let assert_ (self:t) l op c ~lit : unit =
+    let le = Linexp.of_list l in
+    let c = {Constr.le; const=c; op; lits=Bag.return lit} in
+    Log.debugf 10 (fun k->k "(@[sidekick.intsolver.assert@ %a@])" Constr.pp c);
+    BVec.push self.cs c
+
+  (* TODO: check before hand that [t] occurs nowhere else *)
+  let define (self:t) t l : unit =
+    let le = Linexp.of_list l in
+    BVec.push self.defs (t,le)
+
+  (* #### checking #### *)
+
+  module Check_ = struct
+    module LE = Linexp
+
+    type op =
+      | Leq
+      | Lt
+      | Eq
+      | Eq_mod of {
+          prime: Z.t;
+          pow: int;
+        } (* modulo prime^pow *)
+
+    type constr = {
+      le: LE.t;
+      const: Z.t;
+      op: op;
+      lits: lit Bag.t;
+    }
+
+    type state = {
+      mutable rw: LE.t T_map.t; (* rewrite rules *)
+      mutable vars: int T_map.t; (* variables in at least one constraint *)
+      mutable constrs: constr list;
+    }
+    (* main solving state. mutable, but copied for backtracking.
+       invariant: variables in [rw] do not occur anywhere else
+    *)
+
+    (* perform rewriting on the linear expression *)
+    let norm_le (self:state) (le:LE.t) : LE.t =
+      LE.flat_map
+        (fun t -> try T_map.find t self.rw with Not_found -> LE.return t)
+        le
+
+    let[@inline] count_v self t : int = T_map.get_or ~default:0 t self.vars
+    let[@inline] incr_v (self:state) (t:term) : unit =
+      self.vars <- T_map.add t (1 + count_v self t) self.vars
+    let decr_v (self:state) (t:term) : unit =
+      let n = count_v self t - 1 in
+      assert (n >= 0);
+      self.vars <-
+        (if n=0 then T_map.remove t self.vars
+         else T_map.add t n self.vars)
+
+    let add_constr (self:state) (c:constr) =
+      let c = {c with le=norm_le self c.le } in
+      LE.iter (fun t _ -> incr_v self t) c.le;
+      self.constrs <- c :: self.constrs
+
+    let remove_constr (self:state) (c:constr) =
+      LE.iter (fun t _ -> decr_v self t) c.le
+
+    let create (self:t) : state =
+      let state = {
+        vars=T_map.empty;
+        rw=T_map.empty;
+        constrs=[];
+      } in
+      BVec.iter self.defs
+        ~f:(fun (v,le) ->
+            assert (not (T_map.mem v state.rw));
+            state.rw <- T_map.add v (norm_le state le) state.rw);
+      BVec.iter self.cs
+        ~f:(fun (c:Constr.t) ->
+            let {Constr.le; op; const; lits} = c in
+            let op = match op with
+              | Op.Eq -> Eq
+              | Op.Leq -> Leq
+              | Op.Lt -> Lt
+            in
+            let c = {le;const;lits;op} in
+            add_constr state c
+          );
+      state
+
+    let rec solve_rec (self:state) : result =
+      begin match T_map.choose_opt self.vars with
+        | None ->
+          let m = Model.empty in
+          Sat m (* TODO: model *)
+
+        | Some (t, _) ->
+          Log.debugf 30 (fun k->k "(@[intsolver.elim-var@ %a@])" A.pp_term t);
+          assert false
+
+      end
+
+  end
+
+  let check (self:t) : result =
+    Log.debugf 10 (fun k->k "(@[intsolver.check@])");
+    let state = Check_.create self in
+    Check_.solve_rec state
+
+  let _check_invariants _ = ()
+end
diff --git a/src/intsolver/tests/dune b/src/intsolver/tests/dune
new file mode 100644
index 00000000..3f2210d0
--- /dev/null
+++ b/src/intsolver/tests/dune
@@ -0,0 +1,15 @@
+
+(library
+  (name sidekick_test_intsolver)
+  (libraries zarith sidekick.intsolver sidekick.util sidekick.zarith
+             qcheck alcotest))
+
+;(rule
+;  (targets sidekick_test_intsolver.ml)
+;  (enabled_if (>= %{ocaml_version} 4.08.0))
+;  (action (copy test_intsolver.real.ml %{targets})))
+;
+;(rule
+;  (targets sidekick_test_intsolver.ml)
+;  (enabled_if (< %{ocaml_version} 4.08.0))
+;  (action (with-stdout-to %{targets} (echo "let props=[];; let tests=\"intsolver\",[]"))))
diff --git a/src/intsolver/tests/sidekick_test_intsolver.ml b/src/intsolver/tests/sidekick_test_intsolver.ml
new file mode 100644
index 00000000..ed025f36
--- /dev/null
+++ b/src/intsolver/tests/sidekick_test_intsolver.ml
@@ -0,0 +1,359 @@
+
+open CCMonomorphic
+
+module Fmt = CCFormat
+module QC = QCheck
+module Log = Sidekick_util.Log
+let spf = Printf.sprintf
+
+module ZarithZ = Z
+module Z = Sidekick_zarith.Int
+
+module Var = struct
+  include CCInt
+
+  let pp out x = Format.fprintf out "X_%d" x
+
+  let rand n : t QC.arbitrary = QC.make ~print:(Fmt.to_string pp) @@ QC.Gen.(0--n)
+  type lit = int
+  let pp_lit = Fmt.int
+  let not_lit i = Some (- i)
+end
+
+module Var_map = CCMap.Make(Var)
+
+module Solver = Sidekick_intsolver.Make(struct
+    module Z = Z
+    type term = Var.t
+    let pp_term = Var.pp
+    type lit = Var.lit
+    let pp_lit = Var.pp_lit
+    module T_map = Var_map
+  end)
+
+let unwrap_opt_ msg = function
+  | Some x -> x
+  | None -> failwith msg
+
+let rand_n low n : Z.t QC.arbitrary =
+  QC.map ~rev:ZarithZ.to_int Z.of_int QC.(low -- n)
+
+let rand_z = rand_n (-1000) 30_000
+
+module Step = struct
+  module G = QC.Gen
+
+  type linexp = (Z.t * Var.t) list
+
+  type t =
+    | S_new_var of Var.t
+    | S_define of Var.t * (Z.t * Var.t) list
+    | S_leq of linexp * Z.t
+    | S_lt of linexp * Z.t
+    | S_eq of linexp * Z.t
+
+  let pp_le out (le:linexp) =
+    let pp_pair out (n,x) =
+      if Z.equal Z.one n then Var.pp out x
+      else Fmt.fprintf out "%a . %a" Z.pp n Var.pp x in
+    Fmt.fprintf out "(@[%a@])"
+      Fmt.(list ~sep:(return " +@ ") pp_pair) le
+
+  let pp_ out = function
+    | S_new_var v -> Fmt.fprintf out "(@[new-var %a@])" Var.pp v
+    | S_define (v,le) -> Fmt.fprintf out "(@[define %a@ := %a@])" Var.pp v pp_le le
+    | S_leq (le,n) -> Fmt.fprintf out "(@[upper %a <= %a@])" pp_le le Z.pp n
+    | S_lt (le,n) -> Fmt.fprintf out "(@[upper %a < %a@])" pp_le le Z.pp n
+    | S_eq (le,n) -> Fmt.fprintf out "(@[lower %a > %a@])" pp_le le Z.pp n
+
+  (* check that a sequence is well formed *)
+  let well_formed (l:t list) : bool =
+    let rec aux vars = function
+      | [] -> true
+      | S_new_var v :: tl ->
+        not (List.mem v vars) && aux (v::vars) tl
+      | (S_leq (le,_) | S_lt (le,_) | S_eq (le,_)) :: tl ->
+        List.for_all (fun (_,x) -> List.mem x vars) le && aux vars tl
+      | S_define (x,le) :: tl->
+        not (List.mem x vars) &&
+        List.for_all (fun (_,y) -> List.mem y vars) le &&
+        aux (x::vars) tl
+    in
+    aux [] l
+
+  let shrink_step self =
+    let module S = QC.Shrink in
+    match self with
+    | S_new_var _
+    | S_leq _ | S_lt _ | S_eq _ -> QC.Iter.empty
+    | S_define (x, le) ->
+      let open QC.Iter in
+      let* le = S.list le in
+      if List.length le >= 2 then return (S_define (x,le)) else empty
+
+  let rand_steps (n:int) : t list QC.Gen.t =
+    let open G in
+    let rec aux n vars acc =
+      if n<=0 then return (List.rev acc)
+      else (
+        let gen_linexp =
+          let* vars' = G.shuffle_l vars in
+          let* n = 1 -- List.length vars' in
+          let vars' = CCList.take n vars' in
+          assert (List.length vars' = n);
+          let* coeffs = list_repeat n rand_z.gen in
+          return (List.combine coeffs vars')
+        in
+        let* vars, proof_rule =
+          frequency @@ List.flatten [
+            (* add a constraint *)
+            (match vars with
+             | [] -> []
+             | _ ->
+               let gen =
+                 let+ le = gen_linexp
+                 and+ kind = oneofl [`Leq;`Lt;`Eq]
+                 and+ n = rand_z.QC.gen in
+                 vars, (match kind with
+                     | `Lt -> S_lt(le,n)
+                     | `Leq -> S_leq(le,n)
+                     | `Eq -> S_eq(le,n)
+                   )
+               in
+               [6, gen]);
+            (* make a new non-basic var *)
+            (let gen =
+               let v = List.length vars in
+               return ((v::vars), S_new_var v)
+             in
+             [2, gen]);
+            (* make a definition *)
+            (if List.length vars>2
+             then (
+               let v = List.length vars in
+               let gen =
+                 let+ le = gen_linexp in
+                 v::vars, S_define (v, le)
+               in
+               [5, gen]
+            ) else []);
+          ]
+        in
+        aux (n-1) vars (proof_rule::acc)
+      )
+    in
+    aux n [] []
+
+  (* shrink a list but keep it well formed *)
+  let shrink : t list QC.Shrink.t =
+    QC.Shrink.(filter well_formed @@ list ~shrink:shrink_step)
+
+  let gen_for n1 n2 =
+    let open G in
+    assert (n1 < n2);
+    let* n = n1 -- n2 in
+    rand_steps n
+
+  let rand_for n1 n2 : t list QC.arbitrary =
+    let print = Fmt.to_string (Fmt.Dump.list pp_) in
+    QC.make ~shrink ~print (gen_for n1 n2)
+
+  let rand : t list QC.arbitrary = rand_for 1 100
+end
+
+let on_propagate _ ~reason:_ = ()
+
+(* add a single proof_rule to the solvere *)
+let add_step solver (s:Step.t) : unit =
+  begin match s with
+    | Step.S_new_var _v -> ()
+    | Step.S_leq (le,n) ->
+      Solver.assert_ solver le Solver.Op.Leq n ~lit:0
+    | Step.S_lt (le,n) ->
+      Solver.assert_ solver le Solver.Op.Lt n ~lit:0
+    | Step.S_eq (le,n) ->
+      Solver.assert_ solver le Solver.Op.Eq n ~lit:0
+    | Step.S_define (x,le) ->
+      Solver.define solver x le
+  end
+
+let add_steps ?(f=fun()->()) (solver:Solver.t) l : unit =
+  f();
+  List.iter
+    (fun s -> add_step solver s; f())
+    l
+
+(* is this solver's state sat? *)
+let check_solver_is_sat solver : bool =
+  match Solver.check solver with
+  | Solver.Sat _ -> true
+  | Solver.Unsat _ -> false
+
+(* is this problem sat? *)
+let check_pb_is_sat pb : bool =
+  let solver = Solver.create() in
+  add_steps solver pb;
+  check_solver_is_sat solver
+
+(* basic debug printer for Q.t *)
+let str_z n = ZarithZ.to_string n
+
+let prop_sound ?(inv=false) pb =
+  let solver = Solver.create () in
+  begin match
+      add_steps solver pb;
+      Solver.check solver
+    with
+    | Sat model ->
+
+      let get_val v =
+        match Solver.Model.eval model v with
+        | Some n -> n
+        | None -> assert false
+      in
+
+      let eval_le le =
+        List.fold_left (fun s (n,y) -> Z.(s + n * get_val y)) Z.zero le
+      in
+
+      let check_step s =
+        (try
+           if inv then Solver._check_invariants solver;
+           match s with
+           | Step.S_new_var _ -> ()
+           | Step.S_define (x, le) ->
+             let v_x = get_val x in
+             let v_le = eval_le le in
+             if Z.(v_x <> v_le) then (
+               failwith (spf "bad def (X_%d): val(x)=%s, val(expr)=%s" x (str_z v_x)(str_z v_le))
+             );
+           | Step.S_lt (x, n) ->
+             let v_x = eval_le x in
+             if Z.(v_x >= n) then failwith (spf "val=%s, n=%s"(str_z v_x)(str_z n))
+           | Step.S_leq (x, n) ->
+             let v_x = eval_le x in
+             if Z.(v_x > n) then failwith (spf "val=%s, n=%s"(str_z v_x)(str_z n))
+           | Step.S_eq (x, n) ->
+             let v_x = eval_le x in
+             if Z.(v_x <> n) then failwith (spf "val=%s, n=%s"(str_z v_x)(str_z n))
+         with e ->
+           QC.Test.fail_reportf "proof_rule failed: %a@.exn:@.%s@."
+             Step.pp_ s (Printexc.to_string e)
+        );
+        if inv then Solver._check_invariants solver;
+        true
+      in
+      List.for_all check_step pb
+
+    | Solver.Unsat _cert ->
+      (* FIXME:
+         Solver._check_cert cert;
+         *)
+      true
+  end
+
+(* a bunch of useful stats for a problem *)
+let steps_stats = [
+  "n-define", Step.(List.fold_left (fun n -> function S_define _ -> n+1 | _->n) 0);
+  "n-bnd",
+  Step.(List.fold_left
+          (fun n -> function (S_leq _ | S_lt _ | S_eq _) -> n+1 | _->n) 0);
+  "n-vars",
+  Step.(List.fold_left
+          (fun n -> function S_define _ | S_new_var _ -> n+1 | _ -> n) 0);
+]
+
+let enable_stats =
+  match Sys.getenv_opt "TEST_STAT" with Some("1"|"true") -> true | _ -> false
+
+let set_stats_maybe ar =
+  if enable_stats then QC.set_stats steps_stats ar else ar
+
+let check_sound =
+  let ar =
+    Step.(rand_for 0 300)
+    |> QC.set_collect (fun pb -> if check_pb_is_sat pb then "sat" else "unsat")
+    |> set_stats_maybe
+  in
+  QC.Test.make ~long_factor:10 ~count:500 ~name:"solver2_sound" ar prop_sound
+
+let prop_backtrack pb =
+  let solver = Solver.create () in
+  let stack = Stack.create() in
+  let res = ref true in
+  begin try
+    List.iter
+      (fun s ->
+         let is_sat = check_solver_is_sat solver in
+         Solver.push_level solver;
+         Stack.push is_sat stack;
+         if not is_sat then (res := false; raise Exit);
+         add_step solver s;
+      )
+    pb;
+    with Exit -> ()
+  end;
+  res := !res && check_solver_is_sat solver;
+  Log.debugf 50 (fun k->k "res=%b, expected=%b" !res (check_pb_is_sat pb));
+  assert CCBool.(equal !res (check_pb_is_sat pb));
+  (* now backtrack and check at each level *)
+  while not (Stack.is_empty stack) do
+    let res = Stack.pop stack in
+    Solver.pop_levels solver 1;
+    assert CCBool.(equal res (check_solver_is_sat solver))
+  done;
+  true
+
+let check_backtrack =
+  let ar =
+    Step.(rand_for 0 300)
+    |> QC.set_collect (fun pb -> if check_pb_is_sat pb then "sat" else "unsat")
+    |> set_stats_maybe
+  in
+  QC.Test.make
+    ~long_factor:10 ~count:200 ~name:"solver2_backtrack"
+    ar prop_backtrack
+
+let check_scalable =
+  let prop pb =
+    let solver = Solver.create () in
+    add_steps solver pb;
+    ignore (Solver.check solver : Solver.result);
+    true
+  in
+  let ar =
+    Step.(rand_for 3_000 5_000)
+    |> QC.set_collect (fun pb -> if check_pb_is_sat pb then "sat" else "unsat")
+    |> set_stats_maybe
+  in
+  QC.Test.make ~long_factor:2 ~count:10 ~name:"solver2_scalable"
+     ar prop
+
+let props = [
+  check_sound;
+  check_backtrack;
+  check_scalable;
+]
+
+(* regression tests *)
+
+module Reg = struct
+  let alco_mk name f = name, `Quick, f
+
+  let reg_prop_sound ?inv name l =
+    alco_mk name @@ fun () ->
+    if not (prop_sound ?inv l) then Alcotest.fail "fail";
+    ()
+
+  let reg_prop_backtrack name l =
+    alco_mk name @@ fun () ->
+    if not (prop_backtrack l) then Alcotest.fail "fail";
+    ()
+
+  open Step
+  let tests = [
+  ]
+end
+
+let tests =
+  "solver", List.flatten [ Reg.tests ]