add a (CM)RA instance for the unit type

modures/cmra.v

@@ -296,6 +296,9 @@ Section discrete.
 End discrete.
 Arguments discreteRA _ {_ _ _ _ _ _}.
 
+(** CMRA for the unit type *)
+Canonical Structure unitRA : cmraT := discreteRA ().
+
 (** Product *)
 Section prod.
   Context {A B : cmraT}.

modures/ra.v

@@ -231,3 +231,21 @@ Ltac solve_included :=
   ra_reflection.quote;
   apply ra_reflection.flatten_correct, (bool_decide_unpack _);
   vm_compute; apply I.
+
+(** An RA for the unit type *)
+Instance unit_valid : Valid () := λ x, True.
+Instance unit_unit : Unit () := λ x, x.
+Instance unit_op : Op () := λ x y, tt.
+Instance unit_minus : Minus () := λ x y, tt.
+
+Instance unit_ra : RA ().
+Proof.
+  split; done.
+Qed.
+
+
+Instance unit_empty : Empty () := tt.
+Instance unit_empty_ra : RAIdentity().
+Proof.
+  split; done.
+Qed.