Require Export FcEtt.tactics.
Require Export FcEtt.ett_inf.

(* Specifications: *)
Lemma union_notin_iff a s1 s2 :
a `notin` union s1 s2 ↔ a `notin` s1 ∧ a `notin` s2.
Lemma subset_notin : forall x S1 S2,
x `notin` S2 -> S1 [<=] S2 -> x `notin` S1.