diff --git a/lib/bap_sema/bap_sema_flatten.ml b/lib/bap_sema/bap_sema_flatten.ml
index 4e42f1667..1db263af2 100644
--- a/lib/bap_sema/bap_sema_flatten.ml
+++ b/lib/bap_sema/bap_sema_flatten.ml
@@ -2,7 +2,6 @@ open Core_kernel
 open Bap_types.Std
 open Bap_ir
 
-
 let get_direct_typ (e : exp) : Type.t = match e with
   | Bil.Var v -> Var.typ v
   | Bil.Unknown (_,t) -> t
@@ -21,9 +20,15 @@ class substituter (x : var) (x' : var) = object
     Let (v, exp, body)
 end
 
-let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
+let flatten_exp
+    ?(before : tid option = None)
+    (exp : exp)
+    (blk : blk term) : exp * blk term =
   let is_virtual = true in
   let fresh = true in
+  let insert blk def = match before with
+    | None -> Term.append def_t blk def
+    | Some before -> Term.prepend def_t ~before blk def in
   let rec aux (exp : exp) (blk : blk term) = match exp with
     | Bil.Load (x, y, endian, s) ->
       let x, blk = aux x blk in
@@ -32,8 +37,7 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Load (x, y, endian, s) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.Store (x, y, z, endian, s) ->
       let x, blk = aux x blk in
       let y, blk = aux y blk in
@@ -42,8 +46,7 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Store (x, y, z, endian, s) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.BinOp (b, x, y) ->
       let x, blk = aux x blk in
       let y, blk = aux y blk in
@@ -51,26 +54,22 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.BinOp(b, x, y) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.UnOp (u, x) ->
       let x, blk = aux x blk in
       let vtype = get_direct_typ x in
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.UnOp(u, x) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
-    | Bil.Var _
-    | Bil.Int _ -> exp, blk
+      Bil.Var var, insert blk def
+    | Bil.Var _ | Bil.Int _ -> exp, blk
     | Bil.Cast (c, n, x) ->
       let x, blk = aux x blk in
       let vtype = Type.Imm n in
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Cast (c, n, x) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.Let (v, x, y) ->
       let x, blk = aux x blk in
       let var, blk = match x with
@@ -79,7 +78,7 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
           let vtype = Var.typ v in
           let var = Var.create ~is_virtual ~fresh "flt" vtype in
           let def = Ir_def.create var x in
-          var, Term.prepend def_t ~before blk def in
+          var, insert blk def in
       let y = (new substituter v var)#map_exp y in
       aux y blk
     | Bil.Unknown (_, _) -> exp, blk
@@ -91,16 +90,14 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Ite (x, y, z) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.Extract (n, p, x) ->
       let x, blk = aux x blk in
       let vtype = get_direct_typ x in
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Extract (n, p, x) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def
+      Bil.Var var, insert blk def
     | Bil.Concat (x, y) ->
       let x, blk = aux x blk in
       let y, blk = aux y blk in
@@ -108,30 +105,28 @@ let flatten_exp (exp : exp) (blk : blk term) (before : tid) : exp * blk term =
       let var = Var.create ~is_virtual ~fresh "flt" vtype in
       let e = Bil.Concat (x, y) in
       let def = Ir_def.create var e in
-      Bil.Var var,
-      Term.prepend def_t ~before blk def in
+      Bil.Var var, insert blk def in
   aux exp blk
 
 let flatten_blk original_blk =
   let rec flatten_elts (elts : Ir_blk.elt seq) (blk : blk term) =
     let rec flatten_jmp (jmp : Ir_jmp.t) (expseq : exp seq) (blk : blk term) =
       match Seq.next expseq with
-      | Some(hd, tl) ->
-        let exp, blk = flatten_exp hd blk (Term.tid jmp) in
+      | Some (hd, tl) ->
+        let exp, blk = flatten_exp hd blk in
         Ir_jmp.substitute jmp hd exp |> Term.update jmp_t blk |>
         flatten_jmp jmp tl
       | None -> blk in
-
     match Seq.next elts with
-    | Some (hd, tl) -> (match hd with
-        | `Def def ->
-          let exp, blk = flatten_exp (Ir_def.rhs def) blk (Term.tid def) in
-          Ir_def.with_rhs def exp |> Term.update def_t blk |>
-          flatten_elts tl
-        | `Jmp jmp -> flatten_jmp jmp (Ir_jmp.exps jmp) blk
-        | `Phi phi -> flatten_elts tl blk)
-    | None -> blk in
-
+    | None -> blk
+    | Some (hd, tl) -> match hd with
+      | `Def def ->
+        let before = Some (Term.tid def) in
+        let exp, blk = flatten_exp (Ir_def.rhs def) blk ~before in
+        Ir_def.with_rhs def exp |> Term.update def_t blk |>
+        flatten_elts tl
+      | `Jmp jmp -> flatten_jmp jmp (Ir_jmp.exps jmp) blk
+      | `Phi _phi -> flatten_elts tl blk in
   flatten_elts (Ir_blk.elts original_blk) original_blk
 
 let flatten_sub =