diff --git a/heapster-saw/examples/mbox.sawcore b/heapster-saw/examples/mbox.sawcore
index 28b4106695..0b751a6be9 100644
--- a/heapster-saw/examples/mbox.sawcore
+++ b/heapster-saw/examples/mbox.sawcore
@@ -79,4 +79,4 @@ mboxAllFreedSpec : CompM (Vec 1 Bool);
 mboxAllFreedSpec = returnM (Vec 1 Bool) (bvNat 1 0);
 
 randSpec : CompM (Vec 32 Bool);
-randSpec = returnM (Vec 32 Bool) (bvNat 32 3);
+randSpec = existsM (Vec 32 Bool) (Vec 32 Bool) (\ (x : Vec 32 Bool) -> returnM (Vec 32 Bool) x);
diff --git a/heapster-saw/examples/mbox_proofs.v b/heapster-saw/examples/mbox_proofs.v
index 260d132663..089484f85b 100644
--- a/heapster-saw/examples/mbox_proofs.v
+++ b/heapster-saw/examples/mbox_proofs.v
@@ -83,6 +83,59 @@ Hint Rewrite transMbox_Mbox_nil_r transMbox_assoc : refinesM.
 (* ========================================================================== *)
 
 
+Definition mbox_randomize_precond : Mbox -> Prop :=
+  Mbox__rec (fun _ => Prop) True (fun strt len _ _ _ =>
+    (* 0 <= strt <= len < 128 *)
+    isBvsle 64 (intToBv 64 0) strt /\ isBvsle 64 strt len /\
+    isBvslt 64 len (intToBv 64 128)).
+
+Definition mbox_randomize_invar (strt len i : bitvector 64) : Prop :=
+  (* 0 <= strt <= i <= len < 128 *)
+  isBvsle 64 (intToBv 64 0) strt /\ isBvsle 64 strt i /\
+  isBvsle 64 i len /\ isBvslt 64 len (intToBv 64 128).
+
+Lemma no_errors_mbox_randomize
+  : refinesFun mbox_randomize (fun m => assumingM (mbox_randomize_precond m) noErrorsSpec).
+Proof.
+  unfold mbox_randomize, mbox_randomize__tuple_fun, mbox_randomize_precond.
+  prove_refinement_match_letRecM_l.
+  - exact (fun strt len m d i => assumingM (mbox_randomize_invar strt len i) noErrorsSpec).
+  unfold noErrorsSpec, randSpec, mbox_randomize_invar.
+  time "no_errors_mbox_randomize" prove_refinement.
+  (* All the `Mbox_def` and `Vec 32 bool` goals are only every used in *)
+  (* impossible cases, so they can be set to whatever Coq chooses. These *)
+  (* calls to `assumption` also take care of showing that the loop invariant *)
+  (* holds initially from our precondition, and a few of the cases of showing *)
+  (* that the loop invariant holds inductively (see below). *)
+  all: try assumption.
+  (* Showing the error case of the array bounds check is impossible by virtue *)
+  (* of our loop invariant *)
+  - assert (isBvsle 64 (intToBv 64 0) a4) by (rewrite e_assuming0; eauto).
+    assert (isBvsle 64 (intToBv 64 0) a1) by (rewrite H; eauto).
+    apply isBvult_to_isBvslt_pos in e_if; eauto.
+    assert (isBvult 64 a4 (intToBv 64 128)).
+    + apply isBvult_to_isBvslt_pos; [ eauto | reflexivity | ].
+      rewrite e_if; eauto.
+    + rewrite H1 in e_maybe; discriminate e_maybe.
+  (* Showing the loop invariant holds inductively (the remaining two cases) *)
+  - rewrite e_assuming1; apply isBvsle_suc_r.
+    apply isBvslt_to_isBvsle.
+    rewrite e_assuming2, e_assuming3.
+    reflexivity.
+  - apply isBvslt_to_isBvsle_suc.
+    apply isBvult_to_isBvslt_pos in e_if.
+    + assumption.
+    + rewrite e_assuming0; eauto.
+    + rewrite e_assuming0, e_assuming1; eauto.
+  (* Showing the error case of the overflow check is impossible by virtue of *)
+  (* our loop invariant *)
+  - rewrite <- e_assuming1, <- e_assuming0 in e_if0.
+    vm_compute in e_if0; discriminate e_if0.
+  - rewrite e_assuming2, e_assuming3 in e_if0.
+    vm_compute in e_if0; discriminate e_if0.
+Qed.
+
+
 Lemma no_errors_mbox_drop
   : refinesFun mbox_drop (fun _ _ => noErrorsSpec).
 Proof.
diff --git a/saw-core-coq/coq/generated/CryptolToCoq/SAWCorePrelude.v b/saw-core-coq/coq/generated/CryptolToCoq/SAWCorePrelude.v
index 754c2aa84d..68af8d8877 100644
--- a/saw-core-coq/coq/generated/CryptolToCoq/SAWCorePrelude.v
+++ b/saw-core-coq/coq/generated/CryptolToCoq/SAWCorePrelude.v
@@ -1004,6 +1004,8 @@ Definition foldIRT : forall (As : ListSort), forall (Ds : IRTSubsts As), forall
 
 (* Prelude.bindM was skipped *)
 
+(* Prelude.existsM was skipped *)
+
 Definition fmapM : forall (a : Type), forall (b : Type), (a -> b) -> CompM a -> CompM b :=
   fun (a : Type) (b : Type) (f : a -> b) (m : CompM a) => @bindM CompM _ a b m (fun (x : a) => @returnM CompM _ b (f x)).
 
diff --git a/saw-core-coq/src/Verifier/SAW/Translation/Coq/SpecialTreatment.hs b/saw-core-coq/src/Verifier/SAW/Translation/Coq/SpecialTreatment.hs
index 25fc84018f..81d59dd31d 100644
--- a/saw-core-coq/src/Verifier/SAW/Translation/Coq/SpecialTreatment.hs
+++ b/saw-core-coq/src/Verifier/SAW/Translation/Coq/SpecialTreatment.hs
@@ -439,6 +439,7 @@ sawCorePreludeSpecialTreatmentMap configuration =
   , ("catchM",               skip)
   , ("fixM",                 replace (Coq.App (Coq.ExplVar "fixM")
                                        [Coq.Var "CompM", Coq.Var "_"]))
+  , ("existsM",              mapsToExpl compMModule "existsM")
   , ("LetRecType",           mapsTo compMModule "LetRecType")
   , ("LRT_Ret",              mapsTo compMModule "LRT_Ret")
   , ("LRT_Fun",              mapsTo compMModule "LRT_Fun")
diff --git a/saw-core/prelude/Prelude.sawcore b/saw-core/prelude/Prelude.sawcore
index 3191fdd0fb..6dd396f070 100644
--- a/saw-core/prelude/Prelude.sawcore
+++ b/saw-core/prelude/Prelude.sawcore
@@ -1962,6 +1962,7 @@ primitive CompM : sort 0 -> sort 0;
 
 primitive returnM : (a:sort 0) -> a -> CompM a;
 primitive bindM : (a b:sort 0) -> CompM a -> (a -> CompM b) -> CompM b;
+primitive existsM : (a:sort 0) -> (b:sort 0) -> (a -> CompM b) -> CompM b;
 
 -- Apply a pure function to a computation
 fmapM : (a b: sort 0) -> (a -> b) -> CompM a -> CompM b;