(************** Meta rules of existential introduction/elimination  ********)

(* Specifications: *)
Lemma elim_ex : forall a:N, ¬(forall B, ¬(B ε a)) ⊃ exists B:N, (B ε a).
Lemma intro_ex : forall a:N, (exists B:N, (B ε a)) ⊃ ¬(forall B, ¬(B ε a)).
Parameter exist_at_least      : N->Prop.