| n ≤ # x0, ∀ m0, n0. # x0 = m0 + 1 ∧ n0 ≤ m0 → #(x0 \ x0[n0] ') = m0 ∨ x0 = [] ⊦ x0 = [], #(x0 \ (a ' + x0)[n] ') + 1 = # x0, a = (a ' + x0)[n], dups(x0), a ∈ x0 |
n ≤ # x0, ∀ m, n0. # x0 = m + 1 ∧ n0 ≤ m → #(x0 \ x0[n0] ') = m ⊦ x0 = [], #(x0 \ (a ' + x0)[n] ') + 1 = # x0, a = (a ' + x0)[n], dups(x0), a ∈ x0