| ax.toSet ∩ as = ∅ ⊦ #(prefixn(n, ax).toSet ∩ as) = 0, prefixn(n, ax).toSet ∩ as = ∅ |
⊦ prefixn(n, ax) = (n ≤ # ax ⊃ firstn(n, ax);ax)
ax.toSet ∩ as = ∅ ⊦ #((n ≤ # ax ⊃ firstn(n, ax);ax).toSet ∩ as) = 0, (n ≤ # ax ⊃ firstn(n, ax);ax).toSet ∩ as = ∅
Used terms: ax, n