¬ r ∈ rs, r ≠ null ⊦ R[rs, f, g] ↔ (r ==>_ * R)[rs ++ r, f, g]
⊦ (r ==>_ * R)[rs ++ r, f, g] ↔ (a[r =>_] * R)[rs ++ r, f, g(r; a)], r = null, r ∈ rs
Used terms: r, g, f, rs, R