theorem Valid'.rotateL {l} {x : α} {r o₁ o₂} (hl : Valid' o₁ l x) (hr : Valid' x r o₂)
    (H1 : ¬size l + size r ≤ 1) (H2 : delta * size l < size r)
    (H3 : 2 * size r ≤ 9 * size l + 5 ∨ size r ≤ 3) : Valid' o₁ (@rotateL α l x r) o₂ := by
  cases' r with rs rl rx rr; · cases H2
  rw [hr.2.size_eq, Nat.lt_succ_iff] at H2
  rw [hr.2.size_eq] at H3
  replace H3 : 2 * (size rl + size rr) ≤ 9 * size l + 3 ∨ size rl + size rr ≤ 2 :=
    H3.imp (@Nat.le_of_add_le_add_right _ 2 _) Nat.le_of_succ_le_succ
  have H3_0 : size l = 0 → size rl + size rr ≤ 2 := by
    intro l0; rw [l0] at H3
    exact
      (or_iff_right_of_imp fun h => (mul_le_mul_left (by decide)).1 (le_trans h (by decide))).1 H3
  have H3p : size l > 0 → 2 * (size rl + size rr) ≤ 9 * size l + 3 := fun l0 : 1 ≤ size l =>
    (or_iff_left_of_imp <| by intro; linarith).1 H3
  have ablem : ∀ {a b : ℕ}, 1 ≤ a → a + b ≤ 2 → b ≤ 1 := by intros; linarith
  have hlp : size l > 0 → ¬size rl + size rr ≤ 1 := fun l0 hb =>
    absurd (le_trans (le_trans (Nat.mul_le_mul_left _ l0) H2) hb) (by decide)
  rw [Ordnode.rotateL]; split_ifs with h
  · have rr0 : size rr > 0 :=
      (mul_lt_mul_left (by decide)).1 (lt_of_le_of_lt (Nat.zero_le _) h : ratio * 0 < _)
    suffices BalancedSz (size l) (size rl) ∧ BalancedSz (size l + size rl + 1) (size rr) by
      exact hl.node3L hr.left hr.right this.1 this.2
    rcases Nat.eq_zero_or_pos (size l) with l0 | l0
    · rw [l0]; replace H3 := H3_0 l0
      have := hr.3.1
      rcases Nat.eq_zero_or_pos (size rl) with rl0 | rl0
      · rw [rl0] at this ⊢
        rw [le_antisymm (balancedSz_zero.1 this.symm) rr0]
        decide
      have rr1 : size rr = 1 := le_antisymm (ablem rl0 H3) rr0
      rw [add_comm] at H3
      rw [rr1, show size rl = 1 from le_antisymm (ablem rr0 H3) rl0]
      decide
    replace H3 := H3p l0
    rcases hr.3.1.resolve_left (hlp l0) with ⟨_, hb₂⟩
    refine' ⟨Or.inr ⟨_, _⟩, Or.inr ⟨_, _⟩⟩
    · exact Valid'.rotateL_lemma₁ H2 hb₂
    · exact Nat.le_of_lt_succ (Valid'.rotateL_lemma₂ H3 h)
    · exact Valid'.rotateL_lemma₃ H2 h
    · exact
        le_trans hb₂
          (Nat.mul_le_mul_left _ <| le_trans (Nat.le_add_left _ _) (Nat.le_add_right _ _))
  · rcases Nat.eq_zero_or_pos (size rl) with rl0 | rl0
    · rw [rl0, not_lt, le_zero_iff, Nat.mul_eq_zero] at h
      replace h := h.resolve_left (by decide)
      erw [rl0, h, le_zero_iff, Nat.mul_eq_zero] at H2
      rw [hr.2.size_eq, rl0, h, H2.resolve_left (by decide)] at H1
      cases H1 (by decide)
    refine' hl.node4L hr.left hr.right rl0 _
    rcases Nat.eq_zero_or_pos (size l) with l0 | l0
    · replace H3 := H3_0 l0
      rcases Nat.eq_zero_or_pos (size rr) with rr0 | rr0
      · have := hr.3.1
        rw [rr0] at this
        exact Or.inl ⟨l0, le_antisymm (balancedSz_zero.1 this) rl0, rr0.symm ▸ zero_le_one⟩
      exact Or.inl ⟨l0, le_antisymm (ablem rr0 <| by 

Proof state:
α : Type u_1
inst✝ : Preorder α
l : Ordnode α
x : α
o₁ : WithBot α
o₂ : WithTop α
hl : Valid' o₁ l ↑x
rs : ℕ
rl : Ordnode α
rx : α
rr : Ordnode α
hr : Valid' (↑x) (Ordnode.node rs rl rx rr) o₂
H1 : ¬size l + size (Ordnode.node rs rl rx rr) ≤ 1
H2 : delta * size l ≤ size rl + size rr
H3_0 : size l = 0 → size rl + size rr ≤ 2
H3p : size l > 0 → 2 * (size rl + size rr) ≤ 9 * size l + 3
ablem : ∀ {a b : ℕ}, 1 ≤ a → a + b ≤ 2 → b ≤ 1
hlp : size l > 0 → ¬size rl + size rr ≤ 1
h : ¬size rl < ratio * size rr
rl0 : size rl > 0
l0 : size l = 0
H3 : size rl + size rr ≤ 2
rr0 : size rr > 0
⊢ size rr + size rl ≤ 2

Next tactic:
rwa [add_comm]