⊦ key ∈ keys(fs, ax) ↔ (∃ adr. adr ∈ ax ∧ fs[adr].key = key)
ax.toSet = range ri | n, ri-cons(ri, fs), ∃ adr. adr ∈ ax ∧ fs[adr].key = key ⊦ key ∈ ri
Used terms: ax, fs, key