we also need to go below mkSet for \notin

prelude/sets.v

@@ -20,6 +20,8 @@ Proof. by split; [split | |]; repeat intro. Qed.
 
 Lemma mkSet_elem_of {A} (f : A → Prop) x : f x → x ∈ mkSet f.
 Proof. done. Qed.
+Lemma mkSet_not_elem_of {A} (f : A → Prop) x : ~f x → x ∉ mkSet f.
+Proof. done. Qed.
 
 Instance set_ret : MRet set := λ A (x : A), {[ x ]}.
 Instance set_bind : MBind set := λ A B (f : A → set B) (X : set A),