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']
      

Proof state:
case node.node
α : Type u_1
x : α
ls : ℕ
ll : Ordnode α
lx : α
lr : Ordnode α
hl : Balanced (node ls ll lx lr)
sl : Sized (node ls ll lx lr)
rs : ℕ
rl : Ordnode α
rx : α
rr : Ordnode α
hr : Balanced (node rs rl rx rr)
sr : Sized (node rs rl rx rr)
⊢ (if delta * ls < rs then
      rec (motive := fun t => rl = t → Ordnode α) (fun h => nil)
        (fun size l x_1 r l_ih r_ih h =>
          rec (motive := fun t => rr = t → Ordnode α) (fun h => nil)
            (fun size_1 l_1 x_2 r_1 l_ih r_ih h =>
              if size < ratio * size_1 then node (ls + rs + 1) (node (ls + size + 1) (node ls ll lx lr) x rl) rx rr
              else
                node (ls + rs + 1) (node (ls + Ordnode.size l + 1) (node ls ll lx lr) x l) x_1
                  (node (Ordnode.size r + size_1 + 1) r rx rr))
            rr (_ : id rr = id rr))
        rl (_ : id rl = id rl)
    else
      if delta * rs < ls then
        rec (motive := fun t => ll = t → Ordnode α) (fun h => nil)
          (fun size l x_1 r l_ih r_ih h =>
            rec (motive := fun t => lr = 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) ll lx (node (size_1 + rs + 1) lr x (node rs rl rx rr))
                else
                  node (ls + rs + 1) (node (size + Ordnode.size l + 1) ll lx l) x_2
                    (node (Ordnode.size r + rs + 1) r x (node rs rl rx rr)))
              lr (_ : id lr = id lr))
          ll (_ : id ll = id ll)
      else node (ls + rs + 1) (node ls ll lx lr) x (node rs rl rx rr)) =
    if ls + rs ≤ 1 then node' (node ls ll lx lr) x (node rs rl rx rr)
    else
      if delta * ls < rs then rotateL (node ls ll lx lr) x (node rs rl rx rr)
      else
        if delta * rs < ls then rotateR (node ls ll lx lr) x (node rs rl rx rr)
        else node' (node ls ll lx lr) x (node rs rl rx rr)

Next tactic:
symm