diff --git a/_CoqProject b/_CoqProject
index 65799cfc1c5a0f6fab9b2b84cef4518e3b7d9163..0e6aa28f39ccfbb2eca9ce50847d88bf0af474da 100644
--- a/_CoqProject
+++ b/_CoqProject
@@ -24,6 +24,7 @@ theories/lang/notation.v
 theories/lang/proofmode.v
 theories/lang/races.v
 theories/lang/tactics.v
+theories/lang/spawn.v
 theories/typing/type.v
 theories/typing/lft_contexts.v
 theories/typing/type_context.v
diff --git a/theories/lang/spawn.v b/theories/lang/spawn.v
new file mode 100644
index 0000000000000000000000000000000000000000..e62b76ed04653423dcc123fe2feff88a238bb4f7
--- /dev/null
+++ b/theories/lang/spawn.v
@@ -0,0 +1,127 @@
+From iris.program_logic Require Import weakestpre.
+From iris.base_logic.lib Require Import invariants.
+From iris.proofmode Require Import tactics.
+From iris.algebra Require Import excl.
+From lrust.lang Require Import proofmode notation.
+From lrust.lang Require Export lang.
+Set Default Proof Using "Type".
+
+Definition spawn : val :=
+  λ: ["f"],
+    let: "c" := Alloc #2 in
+    "c" <- #0;; (* Initialize "sent" field to 0 (non-atomically). *)
+    Fork ("f" ["c"]);;
+    "c".
+Definition finish : val :=
+  λ: ["c"; "v"],
+    "c" +â‚— #1 <- "v";; (* Store data (non-atomically). *)
+    "c" <-ˢᶜ #1;; (* Signal that we finished (atomically!) *)
+    #().
+Definition join : val :=
+  rec: "join" ["c"] :=
+    if: !ˢᶜ"c" then
+      let: "v" := !("c" +â‚— #1) in (* The thread finished, we can non-atomically load the data. *)
+      Free #2 "c";;
+      "v"
+    else
+      "join" ["c"].
+
+(** The CMRA & functor we need. *)
+Class spawnG Σ := SpawnG { spawn_tokG :> inG Σ (exclR unitC) }.
+Definition spawnΣ : gFunctors := #[GFunctor (exclR unitC)].
+
+Instance subG_spawnΣ {Σ} : subG spawnΣ Σ → spawnG Σ.
+Proof. solve_inG. Qed.
+
+(** Now we come to the Iris part of the proof. *)
+Section proof.
+Context `{!lrustG Σ, !spawnG Σ} (N : namespace).
+
+Definition spawn_inv (γf γj : gname) (c : loc) (Ψ : val → iProp Σ) : iProp Σ :=
+  (own γf (Excl ()) ∗ own γj (Excl ()) ∨
+   ∃ b, c ↦ #(lit_of_bool b) ∗ 
+        match b with false => True%I 
+                   | true => ∃ v, shift_loc c 1 ↦ v ∗ Ψ v ∗ own γf (Excl ())
+        end)%I.
+
+Definition finish_handle (c : loc) (Ψ : val → iProp Σ) : iProp Σ :=
+  (∃ γf γj v, own γf (Excl ()) ∗ shift_loc c 1 ↦ v ∗ inv N (spawn_inv γf γj c Ψ))%I.
+
+Definition join_handle (c : loc) (Ψ : val → iProp Σ) : iProp Σ :=
+  (∃ γf γj, own γj (Excl ()) ∗ † c … 2 ∗ inv N (spawn_inv γf γj c Ψ))%I.
+
+Global Instance spawn_inv_ne n γf γj c :
+  Proper (pointwise_relation val (dist n) ==> dist n) (spawn_inv γf γj c).
+Proof. solve_proper. Qed.
+Global Instance finish_handle_ne n c :
+  Proper (pointwise_relation val (dist n) ==> dist n) (finish_handle c).
+Proof. solve_proper. Qed.
+Global Instance join_handle_ne n c :
+  Proper (pointwise_relation val (dist n) ==> dist n) (join_handle c).
+Proof. solve_proper. Qed.
+
+(** The main proofs. *)
+Lemma spawn_spec (Ψ : val → iProp Σ) e (f : val) :
+  to_val e = Some f →
+  {{{ ∀ c, finish_handle c Ψ -∗ WP f [ #c] {{ _, True }} }}}
+    spawn [e] {{{ c, RET #c; join_handle c Ψ }}}.
+Proof.
+  iIntros (<-%of_to_val Φ) "Hf HΦ". rewrite /spawn /=.
+  wp_let. wp_alloc l vl as "Hl" "H†". wp_let.
+  iMod (own_alloc (Excl ())) as (γf) "Hγf"; first done.
+  iMod (own_alloc (Excl ())) as (γj) "Hγj"; first done.
+  (* TODO: maybe we should get vl as a vector here instead of a separate hypothesis for the length? *)
+  assert (2 = length vl)%nat as Hvl.
+  { apply Nat2Z.inj. done. }
+  destruct vl as [|v0 vl]; first done. destruct vl as [|v1 vl]; first done.
+  destruct vl; last done. clear H Hvl.
+  rewrite heap_mapsto_vec_cons heap_mapsto_vec_singleton.
+  iDestruct "Hl" as "[Hc Hd]". wp_write. clear v0.
+  iMod (inv_alloc N _ (spawn_inv γf γj l Ψ) with "[Hc]") as "#?".
+  { iNext. iRight. iExists false. auto. }
+  (* TODO: Cannot use wp_apply due to curried lemma. *)
+  wp_bind (Fork _). iApply (wp_fork with "[Hγf Hf Hd]").
+  - iNext. iApply "Hf". iExists _, _, _. iFrame. auto.
+  - iIntros "!> _". wp_seq. iApply "HΦ". iExists _, _.
+    iFrame. auto.
+Qed.
+
+Lemma finish_spec (Ψ : val → iProp Σ) c v :
+  {{{ finish_handle c Ψ ∗ Ψ v }}} finish [ #c; v] {{{ RET #(); True }}}.
+Proof.
+  iIntros (Φ) "[Hfin HΨ] HΦ". iDestruct "Hfin" as (γf γj v0) "(Hf & Hd & #?)".
+  wp_lam. wp_op. wp_write.
+  wp_bind (_ <-ˢᶜ _)%E. iInv N as "[[>Hf' _]|Hinv]" "Hclose".
+  { iExFalso. iCombine "Hf" "Hf'" as "Hf". iDestruct (own_valid with "Hf") as "%".
+    auto. }
+  iDestruct "Hinv" as (b) "[>Hc _]". wp_write.
+  iMod ("Hclose" with "[-HΦ]").
+  - iNext. iRight. iExists true. iFrame. iExists _. iFrame.
+  - iModIntro. wp_seq. by iApply "HΦ".
+Qed.
+
+Lemma join_spec (Ψ : val → iProp Σ) c :
+  {{{ join_handle c Ψ }}} join [ #c] {{{ v, RET v; Ψ v }}}.
+Proof.
+  iIntros (Φ) "H HΦ". iDestruct "H" as (γf γj) "(Hj & H† & #?)".
+  iLöb as "IH". (* FIXME: Just wp_rec or even simpl unfolds the IH... *)
+  rewrite {1}(lock join). wp_rec.
+  wp_bind (!ˢᶜ _)%E. iInv N as "[[_ >Hj']|Hinv]" "Hclose".
+  { iExFalso. iCombine "Hj" "Hj'" as "Hj". iDestruct (own_valid with "Hj") as "%".
+    auto. }
+  iDestruct "Hinv" as (b) "[>Hc Ho]". wp_read. destruct b; last first.
+  { iMod ("Hclose" with "[Hc]").
+    - iNext. iRight. iExists _. iFrame.
+    - iModIntro. iApply wp_if. iNext. unlock. iApply ("IH" with "Hj H†").
+      auto. }
+  iDestruct "Ho" as (v) "(Hd & HΨ & Hf)".
+  iMod ("Hclose" with "[Hj Hf]").
+  { iNext. iLeft. iFrame. }
+  iModIntro. iApply wp_if. iNext. wp_op. wp_read. wp_let.
+  iAssert (c ↦∗ [ #true; v])%I with "[Hc Hd]" as "Hc".
+  { rewrite heap_mapsto_vec_cons heap_mapsto_vec_singleton. iFrame. }
+  wp_free; first done. iApply "HΦ". done.
+Qed.
+End proof.
+
+Typeclasses Opaque finish_handle join_handle.