theorem fib_le_of_continuantsAux_b :
    n ≤ 1 ∨ ¬(of v).TerminatedAt (n - 2) → (fib n : K) ≤ ((of v).continuantsAux n).b :=
  Nat.strong_induction_on n
    (by
      intro n IH hyp
      rcases n with (_ | _ | n)
      · simp [fib_add_two, continuantsAux] -- case n = 0
      · simp [fib_add_two, continuantsAux] -- case n = 1
      · let g := of v -- case 2 ≤ n
        have : ¬n + 2 ≤ 1 := by linarith
        have not_terminated_at_n : ¬g.TerminatedAt n := Or.resolve_left hyp this
        obtain ⟨gp, s_ppred_nth_eq⟩ : ∃ gp, g.s.get? n = some gp
        exact Option.ne_none_iff_exists'.mp not_terminated_at_n
        set pconts := g.continuantsAux (n + 1) with pconts_eq
        set ppconts := g.continuantsAux n with ppconts_eq
        -- use the recurrence of `continuantsAux`
        suffices (fib n : K) + fib (n + 1) ≤ gp.a * ppconts.b + gp.b * pconts.b by
          simpa [fib_add_two, add_comm,
            continuantsAux_recurrence s_ppred_nth_eq ppconts_eq pconts_eq]
        -- make use of the fact that `gp.a = 1`
        suffices (fib n : K) + fib (n + 1) ≤ ppconts.b + gp.b * pconts.b by
          simpa [of_part_num_eq_one <| part_num_eq_s_a s_ppred_nth_eq]
        have not_terminated_at_pred_n : ¬g.TerminatedAt (n - 1) :=
          mt (terminated_stable <| Nat.sub_le n 1) not_terminated_at_n
        have not_terminated_at_ppred_n : ¬TerminatedAt g (n - 2) :=
          mt (terminated_stable (n - 1).pred_le) not_terminated_at_pred_n
        -- use the IH to get the inequalities for `pconts` and `ppconts`
        have ppred_nth_fib_le_ppconts_B : (fib n : K) ≤ ppconts.b :=
          IH n (lt_trans (Nat.lt.base n) <| Nat.lt.base <| n + 1) (Or.inr not_terminated_at_ppred_n)
        suffices : (fib (n + 1) : K) ≤ gp.b * pconts.b
        solve_by_elim [_root_.add_le_add ppred_nth_fib_le_ppconts_B]
        -- finally use the fact that `1 ≤ gp.b` to solve the goal
        suffices 1 * (fib (n + 1) : K) ≤ gp.b * pconts.b by rwa [one_mul] at this
        have one_le_gp_b : (1 : K) ≤ gp.b :=
          of_one_le_get?_part_denom (part_denom_eq_s_b s_ppred_nth_eq)
        have : (0 : K) ≤ fib (n + 1) := mod_cast (fib (n + 1)).zero_le
        have : (0 : K) ≤ gp.b := le_trans zero_le_one one_le_gp_b
        mono
        · norm_num
        · 

Proof state:
case this.h₂.a
K : Type u_1
v : K
n✝ : ℕ
inst✝¹ : LinearOrderedField K
inst✝ : FloorRing K
n : ℕ
IH : ∀ m < Nat.succ (Nat.succ n), m ≤ 1 ∨ ¬TerminatedAt (of v) (m - 2) → ↑(fib m) ≤ (continuantsAux (of v) m).b
hyp : Nat.succ (Nat.succ n) ≤ 1 ∨ ¬TerminatedAt (of v) (Nat.succ (Nat.succ n) - 2)
g : GeneralizedContinuedFraction K := of v
this✝¹ : ¬n + 2 ≤ 1
not_terminated_at_n : ¬TerminatedAt g n
gp : Pair K
s_ppred_nth_eq : Stream'.Seq.get? g.s n = some gp
pconts : Pair K := continuantsAux g (n + 1)
pconts_eq : pconts = continuantsAux g (n + 1)
ppconts : Pair K := continuantsAux g n
ppconts_eq : ppconts = continuantsAux g n
not_terminated_at_pred_n : ¬TerminatedAt g (n - 1)
not_terminated_at_ppred_n : ¬TerminatedAt g (n - 2)
ppred_nth_fib_le_ppconts_B : ↑(fib n) ≤ ppconts.b
one_le_gp_b : 1 ≤ gp.b
this✝ : 0 ≤ ↑(fib (n + 1))
this : 0 ≤ gp.b
s_ppred_nth_eq_symm : some gp = Stream'.Seq.get? g.s n
⊢ n + 1 ≤ 1 ∨ ¬TerminatedAt (of v) (n + 1 - 2)

Next tactic:
tauto