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+Require Import Test.base.
+
+Lemma dec_stable `{Decision P} : ¬¬P → P.
+Proof. firstorder. Qed.
+
+(** The tactic [destruct_decide] destructs a sumbool [dec]. If one of the
+components is double negated, it will try to remove the double negation. *)
+Tactic Notation "destruct_decide" constr(dec) "as" ident(H) :=
+  destruct dec as [H|H];
+  try match type of H with
+  | ¬¬_ => apply dec_stable in H
+  end.
+Tactic Notation "destruct_decide" constr(dec) :=
+  let H := fresh in destruct_decide dec as H.
+
+
+(** * Monadic operations *)
+Instance option_guard: MGuard option := λ P dec A f,
+  match dec with left H => f H | _ => None end.
+
+(** * Tactics *)
+Tactic Notation "case_option_guard" "as" ident(Hx) :=
+  match goal with
+  | H : context C [@mguard option _ ?P ?dec] |- _ =>
+    change (@mguard option _ P dec) with (λ A (f : P → option A),
+      match @decide P dec with left H' => f H' | _ => None end) in *;
+    destruct_decide (@decide P dec) as Hx
+  | |- context C [@mguard option _ ?P ?dec] =>
+    change (@mguard option _ P dec) with (λ A (f : P → option A),
+      match @decide P dec with left H' => f H' | _ => None end) in *;
+    destruct_decide (@decide P dec) as Hx
+  end.
+Tactic Notation "case_option_guard" :=
+  let H := fresh in case_option_guard as H.
+
+(* This proof failed depending on the name of the module. *)
+Lemma option_guard_True {A} P `{Decision P} (mx : option A) :
+  P → (guard P; mx) = mx.
+Proof. intros. case_option_guard. reflexivity. contradiction. Qed.