| ⊦ datakeys(ks ++ key, ri) = datakeys(ks, ri) ∪ datakeys(key, ri) |
⊦ ks = ks0 ↔ (∀ key. key ∈ ks ↔ key ∈ ks0)
⊦ ∀ key0. key0 ∈ datakeys(ks ++ key, ri) ↔ key0 ∈ datakeys(ks, ri) ∪ datakeys(key, ri)
Used terms: datakeys(ks, ri) ∪ datakeys(key, ri), datakeys(ks ++ key, ri)