⊦ rs = rs0 ↔ (∀ r. r ∈ rs ↔ r ∈ rs0)
∀ r. ¬ r ∈ rs0 ∨ ¬ r ∈ rs1 ⊦ #(rs0 ∩ rs1) = 0, ∀ r. r ∈ rs0 ∩ rs1 ↔ r ∈ ∅
Used terms: ∅, rs0 ∩ rs1