| ⊦ (∃ a. (a[r =>_] * R)[rs ++ r, f, g]) ↔ (a[r =>_] * R)[rs ++ r, f, g(r; a)], r = null, r ∈ rs |
∃ a. (a[r =>_] * R)[rs ++ r, f, g] ⊦ r = null, (a[r =>_] * R)[rs ++ r, f, g(r; a)], r ∈ rs
(a[r =>_] * R)[rs ++ r, f, g(r; a)] ⊦ r = null, r ∈ rs, ∃ a. (a[r =>_] * R)[rs ++ r, f, g]