| ⊦ (ri -- key0) |≤ key1 = (ri |≤ key1) -- key0, 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) -- key0) ∧ (key ∈ (ri -- key0) |≤ key1 → ((ri -- key0) |≤ key1)[key] = ((ri |≤ key1) -- key0)[key])
Used terms: (ri |≤ key1) -- key0, (ri -- key0) |≤ key1