| # ay ≤ # ax0, unique_al(ax0.rest), a ∈ ay, ∀ a0. a0 ∈ ax0 → a0 ∈ ay ⊦ ax0 = [], a ∈ ax0, ax0.first.1 ∈ ax0.rest, ∃ ay0. # ay0 < # ax0 ∧ (∀ a. a ∈ ax0 → a ∈ ay0) |
⊦ a ∈ ax ↔ (∃ d, ay, az. ax = ay + (a × d)' + az)
# ay ≤ # ax0, unique_al(ax0.rest), ∃ d, ay0, az. ay = ay0 + (a × d)' + az, ∀ a0. a0 ∈ ax0 → a0 ∈ ay ⊦ ax0 = [], a ∈ ax0, ax0.first.1 ∈ ax0.rest, ∃ ay0. # ay0 < # ax0 ∧ (∀ a. a ∈ ax0 → a ∈ ay0)
Used terms: ay, a