⊦ zh = zh0 ↔ (∀ r. (r ∈ zh ↔ r ∈ zh0) ∧ (r ∈ zh → zh[r] = zh0[r]))
∅ = zh0 ∪ zh1, zh0 ⊥ zh1 ⊦ ¬ r ∈ zh0 ∧ (∀ r. (r ∈ zh1 ↔ r ∈ ∅) ∧ (r ∈ zh1 → zh1[r] = (∅)[r]))
Used terms: ∅, zh1