Section CONSTRUCTIVE_FACTS.
Variant Th (X : ensemble formula) (b : formula) : Prop :=
| In_Th
(INFERS : X ⊢ b)
: b \in Th X.
#[local] Hint Constructors Th : core.
#[global]
Add Parametric Morphism :
Th with signature (eqProp ==> eqProp)
as Th_lifts_eqProp.

Lemma lemma1_of_1_3_8 (X : ensemble formula)
: isFilter (Th X).