| key ∈ keys(fs, ax0), ∀ key, fs. key ∈ keys(fs, ax0) ↔ (∃ adr. adr ∈ ax0 ∧ fs[adr].key = key) ⊦ key ∈ keys(fs, ax0) ↔ (∃ adr0. adr0 ∈ ax0 ∧ fs[adr0].key = key), key = fs[adr].key |
key ∈ keys(fs, ax0), ∀ key, fs. key ∈ keys(fs, ax0) ↔ (∃ adr. adr ∈ ax0 ∧ fs[adr].key = key) ⊦ key = fs[adr].key, ∃ adr0. adr0 ∈ ax0 ∧ fs[adr0].key = key