-- Lean 4 proof step from Mathlib.Data.Ordmap.Invariants (goal)
Goal:
α : Type u_1
x : α
rs : ℕ
rl : Ordnode α
rx : α
rr : Ordnode α
ls : ℕ
ll : Ordnode α
lx : α
lr : Ordnode α
pos :
  ls > delta * rs →
    Eq.{u_1 + 1} (α := Ordnode α)
      (dual
        (rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
          (fun (size : ℕ) (l : Ordnode α) (x_1 : α) (r l_ih r_ih : Ordnode α) ↦
            rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
              (fun (size_1 : ℕ) (l : Ordnode α) (x_2 : α) (r l_ih r_ih : Ordnode α) ↦
                if size_1 < ratio * size then node (ls + rs + 1) ll lx (node (rs + size_1 + 1) lr x (node rs rl rx rr))
                else
                  node (ls + rs + 1) (node (size + l.size + 1) ll lx l) x_2
                    (node (r.size + rs + 1) r x (node rs rl rx rr)))
              lr)
          ll))
      (rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
        (fun (size : ℕ) (l : Ordnode α) (x_1 : α) (r l_ih r_ih : Ordnode α) ↦
          rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
            (fun (size_1 : ℕ) (l : Ordnode α) (x_2 : α) (r l_ih r_ih : Ordnode α) ↦
              if size_1 < ratio * size then
                node (rs + ls + 1) (node (rs + size_1 + 1) (node rs rr.dual rx rl.dual) x lr.dual) lx ll.dual
              else
                node (rs + ls + 1) (node (rs + l.size + 1) (node rs rr.dual rx rl.dual) x l) x_2
                  (node (r.size + size + 1) r lx ll.dual))
            lr.dual)
        ll.dual)
⊢ Eq.{u_1 + 1} (α := Ordnode α)
    (dual
      (if ls > delta * rs then
        rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
          (fun (size : ℕ) (l : Ordnode α) (x_1 : α) (r l_ih r_ih : Ordnode α) ↦
            rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
              (fun (size_1 : ℕ) (l : Ordnode α) (x_2 : α) (r l_ih r_ih : Ordnode α) ↦
                if size_1 < ratio * size then node (ls + rs + 1) ll lx (node (rs + size_1 + 1) lr x (node rs rl rx rr))
                else
                  node (ls + rs + 1) (node (size + l.size + 1) ll lx l) x_2
                    (node (r.size + rs + 1) r x (node rs rl rx rr)))
              lr)
          ll
      else node (ls + rs + 1) (node ls ll lx lr) x (node rs rl rx rr)))
    (if ls > delta * rs then
      rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
        (fun (size : ℕ) (l : Ordnode α) (x_1 : α) (r l_ih r_ih : Ordnode α) ↦
          rec (motive := fun (x : Ordnode α) ↦ Ordnode α) nil
            (fun (size_1 : ℕ) (l : Ordnode α) (x_2 : α) (r l_ih r_ih : Ordnode α) ↦
              if size_1 < ratio * size then
                node (rs + ls + 1) (node (rs + size_1 + 1) (node rs rr.dual rx rl.dual) x lr.dual) lx ll.dual
              else
                node (rs + ls + 1) (node (rs + l.size + 1) (node rs rr.dual rx rl.dual) x l) x_2
                  (node (r.size + size + 1) r lx ll.dual))
            lr.dual)
        ll.dual
    else node (rs + ls + 1) (node rs rr.dual rx rl.dual) x (node ls lr.dual lx ll.dual))
have neg : ¬ls > delta * rs →
  (node (ls + rs + 1) (node ls ll lx lr) x (node rs rl rx rr)).dual =
    node (rs + ls + 1) (node rs rr.dual rx rl.dual) x (node ls lr.dual lx ll.dual)
Tactic: split_ifs