theorem eq_of_xn_modEq_lem3 {i n} (npos : 0 < n) :
    ∀ {j}, i < j → j ≤ 2 * n → j ≠ n → ¬(a = 2 ∧ n = 1 ∧ i = 0 ∧ j = 2) →
        xn a1 i % xn a1 n < xn a1 j % xn a1 n
  | 0, ij, _, _, _ => absurd ij (Nat.not_lt_zero _)
  | j + 1, ij, j2n, jnn, ntriv =>
    have lem2 : ∀ k > n, k ≤ 2 * n → (↑(xn a1 k % xn a1 n) : ℤ) =
        xn a1 n - xn a1 (2 * n - k) := fun k kn k2n => by
      let k2nl :=
        lt_of_add_lt_add_right <|
          show 2 * n - k + k < n + k by
            rw [tsub_add_cancel_of_le]
            rw [two_mul]; exact add_lt_add_left kn n
            exact k2n
      have xle : xn a1 (2 * n - k) ≤ xn a1 n := le_of_lt <| strictMono_x a1 k2nl
      suffices xn a1 k % xn a1 n = xn a1 n - xn a1 (2 * n - k) by rw [this, Int.ofNat_sub xle]
      rw [← Nat.mod_eq_of_lt (Nat.sub_lt (x_pos a1 n) (x_pos a1 (2 * n - k)))]
      apply ModEq.add_right_cancel' (xn a1 (2 * n - k))
      rw [tsub_add_cancel_of_le xle]
      have t := xn_modEq_x2n_sub_lem a1 k2nl.le
      rw [tsub_tsub_cancel_of_le k2n] at t
      exact t.trans dvd_rfl.zero_modEq_nat
    (lt_trichotomy j n).elim (fun jn : j < n => eq_of_xn_modEq_lem1 _ ij (lt_of_le_of_ne jn jnn))
      fun o =>
      o.elim
        (fun jn : j = n => by
          cases jn
          apply Int.lt_of_ofNat_lt_ofNat
          rw [lem2 (n + 1) (Nat.lt_succ_self _) j2n,
            show 2 * n - (n + 1) = n - 1 by
              rw [two_mul, tsub_add_eq_tsub_tsub, add_tsub_cancel_right]]
          refine' lt_sub_left_of_add_lt (Int.ofNat_lt_ofNat_of_lt _)
          rcases lt_or_eq_of_le <| Nat.le_of_succ_le_succ ij with lin | ein
          · rw [Nat.mod_eq_of_lt (strictMono_x _ lin)]
            have ll : xn a1 (n - 1) + xn a1 (n - 1) ≤ xn a1 n := by
              rw [← two_mul, mul_comm,
                show xn a1 n = xn a1 (n - 1 + 1) by rw [tsub_add_cancel_of_le (succ_le_of_lt npos)],
                xn_succ]
              exact le_trans (Nat.mul_le_mul_left _ a1) (Nat.le_add_right _ _)
            have npm : (n - 1).succ = n := Nat.succ_pred_eq_of_pos npos
            have il : i ≤ n - 1 := by
              apply Nat.le_of_succ_le_succ
              rw [npm]
              exact lin
            rcases lt_or_eq_of_le il with ill | ile
            · exact lt_of_lt_of_le (Nat.add_lt_add_left (strictMono_x a1 ill) _) ll
            · rw [ile]
              apply lt_of_le_of_ne ll
              rw [← two_mul]
              exact fun e =>
                ntriv <| by
                  let ⟨a2, s1⟩ :=
                    @eq_of_xn_modEq_lem2 _ a1 (n - 1)
                      (by rwa [tsub_add_cancel_of_le (succ_le_of_lt npos)])
                  have n1 : n = 1 := le_antisymm (tsub_eq_zero_iff_le.mp s1) npos
                  rw [ile, a2, n1]; exact ⟨rfl, rfl, rfl, rfl⟩
          · rw [ein, Nat.mod_self, add_zero]
            exact strictMono_x _ (Nat.pred_lt npos.ne'))
        fun jn : j > n =>
        have lem1 : j ≠ n → xn a1 j % xn a1 n < xn a1 (j + 1) % xn a1 n →
            xn a1 i % xn a1 n < xn a1 (j + 1) % xn a1 n :=
          fun jn s =>
          (lt_or_eq_of_le (Nat.le_of_succ_le_succ ij)).elim
            (fun h =>
              lt_trans
                (eq_of_xn_modEq_lem3 npos h (le_of_lt (Nat.lt_of_succ_le j2n)) jn
                    fun ⟨a1, n1, i0, j2⟩ => by
                      rw [n1, j2] at j2n; exact absurd j2n (by decide))
                s)
            fun h => by rw [h]; exact s
        lem1 (_root_.ne_of_gt jn) <|
          Int.lt_of_ofNat_lt_ofNat <| by
            rw [lem2 j jn (le_of_lt j2n), lem2 (j + 1) (Nat.le_succ_of_le jn) j2n]
            refine' sub_lt_sub_left (Int.ofNat_lt_ofNat_of_lt <| strictMono_x _ _) _
            rw [Nat.sub_succ]
            

Proof state:
a : ℕ
a1 : 1 < a
i n : ℕ
npos : 0 < n
j : ℕ
ij : i < j + 1
j2n : j + 1 ≤ 2 * n
jnn : j + 1 ≠ n
ntriv : ¬(a = 2 ∧ n = 1 ∧ i = 0 ∧ j + 1 = 2)
lem2 : ∀ k > n, k ≤ 2 * n → ↑(xn a1 k % xn a1 n) = ↑(xn a1 n) - ↑(xn a1 (2 * n - k))
o : j = n ∨ n < j
jn : j > n
lem1 : j ≠ n → xn a1 j % xn a1 n < xn a1 (j + 1) % xn a1 n → xn a1 i % xn a1 n < xn a1 (j + 1) % xn a1 n
⊢ pred (2 * n - j) < 2 * n - j

Next tactic:
exact Nat.pred_lt (_root_.ne_of_gt <| tsub_pos_of_lt j2n)