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