∀ r. r ∈ rs → g(r) ≠ a ⊦ r = null, owns-one(a, r, rs ++ r, g(r; a)), r ∈ rs
Used simplifier rules: owns-none: ⊦ owns-none(a, rs, g) ↔ (∀ r. r ∈ rs → g(r) ≠ a);