Revert git:00444b07 as the weaker notion of atomicity is too weak for unsafe WP.

theories/program_logic/language.v

@@ -82,8 +82,6 @@ Section language.
   Proof. unfold reducible, irreducible. naive_solver. Qed.
   Lemma reducible_not_val e σ : reducible e σ → to_val e = None.
   Proof. intros (?&?&?&?); eauto using val_stuck. Qed.
-  Lemma val_irreducible e σ : is_Some (to_val e) → irreducible e σ.
-  Proof. intros [??] ??? ?%val_stuck. by destruct (to_val e). Qed.
   Global Instance of_val_inj : Inj (=) (=) (@of_val Λ).
   Proof. by intros v v' Hv; apply (inj Some); rewrite -!to_of_val Hv. Qed.