theorem balance_eq_balance' {l x r} (hl : Balanced l) (hr : Balanced r) (sl : Sized l)
    (sr : Sized r) : @balance α l x r = balance' l x r := by
  cases' l with ls ll lx lr
  · cases' r with rs rl rx rr
    · rfl
    · rw [sr.eq_node'] at hr ⊢
      cases' rl with rls rll rlx rlr <;> cases' rr with rrs rrl rrx rrr <;>
        dsimp [balance, balance']
      · rfl
      · have : size rrl = 0 ∧ size rrr = 0 := by
          have := balancedSz_zero.1 hr.1.symm
          rwa [size, sr.2.2.1, Nat.succ_le_succ_iff, le_zero_iff, add_eq_zero_iff] at this
        cases sr.2.2.2.1.size_eq_zero.1 this.1
        cases sr.2.2.2.2.size_eq_zero.1 this.2
        obtain rfl : rrs = 1 := sr.2.2.1
        rw [if_neg, if_pos, rotateL, if_pos]; · rfl
        all_goals dsimp only [size]; decide
      · have : size rll = 0 ∧ size rlr = 0 := by
          have := balancedSz_zero.1 hr.1
          rwa [size, sr.2.1.1, Nat.succ_le_succ_iff, le_zero_iff, add_eq_zero_iff] at this
        cases sr.2.1.2.1.size_eq_zero.1 this.1
        cases sr.2.1.2.2.size_eq_zero.1 this.2
        obtain rfl : rls = 1 := sr.2.1.1
        rw [if_neg, if_pos, rotateL, if_neg]; · rfl
        all_goals dsimp only [size]; decide
      · symm; rw [zero_add, if_neg, if_pos, rotateL]
        · dsimp only [size_node]; split_ifs
          · simp [node3L, node']; abel
          · simp [node4L, node', sr.2.1.1]; abel
        · apply Nat.zero_lt_succ
        · exact not_le_of_gt (Nat.succ_lt_succ (add_pos sr.2.1.pos sr.2.2.pos))
  · cases' r with rs rl rx rr
    · rw [sl.eq_node'] at hl ⊢
      cases' ll with lls lll llx llr <;> cases' lr with lrs lrl lrx lrr <;>
        dsimp [balance, balance']
      · rfl
      · have : size lrl = 0 ∧ size lrr = 0 := by
          have := balancedSz_zero.1 hl.1.symm
          rwa [size, sl.2.2.1, Nat.succ_le_succ_iff, le_zero_iff, add_eq_zero_iff] at this
        cases sl.2.2.2.1.size_eq_zero.1 this.1
        cases sl.2.2.2.2.size_eq_zero.1 this.2
        obtain rfl : lrs = 1 := sl.2.2.1
        rw [if_neg, if_neg, if_pos, rotateR, if_neg]; · rfl
        all_goals dsimp only [size]; decide
      · have : size lll = 0 ∧ size llr = 0 := by
          have := balancedSz_zero.1 hl.1
          rwa [size, sl.2.1.1, Nat.succ_le_succ_iff, le_zero_iff, add_eq_zero_iff] at this
        cases sl.2.1.2.1.size_eq_zero.1 this.1
        cases sl.2.1.2.2.size_eq_zero.1 this.2
        obtain rfl : lls = 1 := sl.2.1.1
        rw [if_neg, if_neg, if_pos, rotateR, if_pos]; · rfl
        all_goals dsimp only [size]; decide
      · symm; rw [if_neg, if_neg, if_pos, rotateR]
        · dsimp only [size_node]; split_ifs
          · simp [node3R, node']; abel
          · simp [node4R, node', sl.2.2.1]; abel
        · apply Nat.zero_lt_succ
        · apply Nat.not_lt_zero
        · exact not_le_of_gt (Nat.succ_lt_succ (add_pos sl.2.1.pos sl.2.2.pos))
    · simp [balance, balance']
      symm; rw [if_neg]
      · split_ifs with h h_1
        · have rd : delta ≤ size rl + size rr := by
            have := lt_of_le_of_lt (Nat.mul_le_mul_left _ sl.pos) h
            rwa [sr.1, Nat.lt_succ_iff] at this
          cases' rl with rls rll rlx rlr
          · rw [size, zero_add] at rd
            exact absurd (le_trans rd (balancedSz_zero.1 hr.1.symm)) (by decide)
          cases' rr with rrs rrl rrx rrr
          · exact absurd (le_trans rd (balancedSz_zero.1 hr.1)) (by decide)
          dsimp [rotateL]; split_ifs
          · simp [node3L, node', sr.1]; abel
          · simp [node4L, node', sr.1, sr.2.1.1]; abel
        · have ld : delta ≤ size ll + size lr := by
            have := lt_of_le_of_lt (Nat.mul_le_mul_left _ sr.pos) h_1
            rwa [sl.1, Nat.lt_succ_iff] at this
          cases' ll with lls lll llx llr
          · rw [size, zero_add] at ld
            exact absurd (le_trans ld (balancedSz_zero.1 hl.1.symm)) (by decide)
          cases' lr with lrs lrl lrx lrr
          · exact absurd (le_trans ld (balancedSz_zero.1 hl.1)) (by decide)
          

Proof state:
case pos.node.node
α : Type u_1
x : α
ls : ℕ
lx : α
rs : ℕ
rl : Ordnode α
rx : α
rr : Ordnode α
hr : Balanced (node rs rl rx rr)
sr : Sized (node rs rl rx rr)
h : ¬delta * ls < rs
h_1 : delta * rs < ls
lls : ℕ
lll : Ordnode α
llx : α
llr : Ordnode α
lrs : ℕ
lrl : Ordnode α
lrx : α
lrr : Ordnode α
hl : Balanced (node ls (node lls lll llx llr) lx (node lrs lrl lrx lrr))
sl : Sized (node ls (node lls lll llx llr) lx (node lrs lrl lrx lrr))
ld : delta ≤ size (node lls lll llx llr) + size (node lrs lrl lrx lrr)
⊢ rotateR (node ls (node lls lll llx llr) lx (node lrs lrl lrx lrr)) x (node rs rl rx rr) =
    rec (motive := fun t => node lls lll llx llr = t → Ordnode α) (fun h => nil)
      (fun size l x_1 r l_ih r_ih h =>
        rec (motive := fun t => node lrs lrl lrx lrr = t → Ordnode α) (fun h => nil)
          (fun size_1 l x_2 r l_ih r_ih h =>
            if size_1 < ratio * size then
              node (ls + rs + 1) (node lls lll llx llr) lx
                (node (size_1 + rs + 1) (node lrs lrl lrx lrr) x (node rs rl rx rr))
            else
              node (ls + rs + 1) (node (size + Ordnode.size l + 1) (node lls lll llx llr) lx l) x_2
                (node (Ordnode.size r + rs + 1) r x (node rs rl rx rr)))
          (node lrs lrl lrx lrr) (_ : id (node lrs lrl lrx lrr) = id (node lrs lrl lrx lrr)))
      (node lls lll llx llr) (_ : id (node lls lll llx llr) = id (node lls lll llx llr))

Next tactic:
dsimp [rotateR]