diff --git a/_CoqProject b/_CoqProject
index 3f8cde4123e745fa55d2e7adae3a80544c7cc3f0..700bd296af444bdac74621d4e55bd887737e29c8 100644
--- a/_CoqProject
+++ b/_CoqProject
@@ -47,6 +47,7 @@ algebra/base.v
 algebra/dra.v
 algebra/cofe_solver.v
 algebra/agree.v
+algebra/dec_agree.v
 algebra/excl.v
 algebra/iprod.v
 algebra/functor.v
diff --git a/algebra/dec_agree.v b/algebra/dec_agree.v
new file mode 100644
index 0000000000000000000000000000000000000000..9683f606f41147a6e0038a8e63cd39fe76955706
--- /dev/null
+++ b/algebra/dec_agree.v
@@ -0,0 +1,49 @@
+From algebra Require Export cmra.
+From algebra Require Import functor upred.
+Local Arguments validN _ _ _ !_ /.
+Local Arguments valid _ _  !_ /.
+Local Arguments op _ _ _ !_ /.
+Local Arguments unit _ _ !_ /.
+
+(* This is isomorphic to optiob, but has a very different RA structure. *)
+Inductive dec_agree (A : Type) : Type := 
+  | DecAgree : A → dec_agree A
+  | DecAgreeBot : dec_agree A.
+Arguments DecAgree {_} _.
+Arguments DecAgreeBot {_}.
+
+Section dec_agree.
+Context {A : Type} `{∀ x y : A, Decision (x = y)}.
+
+Instance dec_agree_valid : Valid (dec_agree A) := λ x,
+  if x is DecAgree _ then True else False.
+Instance dec_agree_equiv : Equiv (dec_agree A) := equivL.
+Canonical Structure dec_agreeC : cofeT := leibnizC (dec_agree A).
+
+Instance dec_agree_op : Op (dec_agree A) := λ x y,
+  match x, y with
+  | DecAgree a, DecAgree b => if decide (a = b) then DecAgree a else DecAgreeBot
+  | _, _ => DecAgreeBot
+  end.
+Instance dec_agree_unit : Unit (dec_agree A) := id.
+Instance dec_agree_minus : Minus (dec_agree A) := λ x y, x.
+
+Definition dec_agree_ra : RA (dec_agree A).
+Proof.
+  split.
+  - apply _.
+  - apply _.
+  - apply _.
+  - apply _.
+  - intros [?|] [?|] [?|]; simpl; repeat (case_match; simpl); subst; congruence.
+  - intros [?|] [?|]; simpl; repeat (case_match; simpl); try subst; congruence.
+  - intros [?|]; simpl; repeat (case_match; simpl); try subst; congruence.
+  - intros [?|]; simpl; repeat (case_match; simpl); try subst; congruence.
+  - intros [?|] [?|] ?; simpl; done.
+  - intros [?|] [?|] ?; simpl; done.
+  - intros [?|] [?|] [[?|]]; simpl; repeat (case_match; simpl); subst;
+      try congruence; [].
+    case=>EQ. destruct EQ. done.
+Qed.
+
+Canonical Structure dec_agreeRA : cmraT := discreteRA dec_agree_ra.