key ∈ ks0, ∀ key. key ∈ ks0 ↔ key ∈ ks1 ⊦ # ks0 = # ks1, key ∈ ks1
key ∈ ks1, ∀ key. key ∈ ks0 ↔ key ∈ ks1 ⊦ # ks0 = # ks1, key ∈ ks0