∀ key. key ∈ ks ↔ key ∈ ks0 ⊦ # ks = # ks0 ↔ true
∀ key. key ∈ ks0 ↔ key ∈ ks1 ⊦ # ks0 = # ks1, ∀ key. key ∈ ks0 ↔ key ∈ ks1
Used terms: ks1, ks0