⊦ inv(a, Pelem) ↔ (∀ b. p*(a, b) → Pelem(b))
Pelem(a), ∀ a, b. Pelem(a) ∧ p(a, b) → Pelem(b) ⊦ ∀ b. p*(a, b) → Pelem(b)
Used terms: Pelem, a