| ⊦ (ri -- key0) |> key1 = ri |> key1, key1 < key0 |
⊦ ri = ri0 ↔ (∀ key. (key ∈ ri ↔ key ∈ ri0) ∧ (key ∈ ri → ri[key] = ri0[key]))
⊦ key1 < key0, ∀ key. (key ∈ (ri -- key0) |> key1 ↔ key ∈ ri |> key1) ∧ (key ∈ (ri -- key0) |> key1 → ((ri -- key0) |> key1)[key] = (ri |> key1)[key])
Used terms: ri |> key1, (ri -- key0) |> key1