Module: Mathlib.Data.Ordmap.Invariants

Goal state before:
pp: case nil.node
α : Type u_1
x : α
ls : ℕ
ll : Ordnode α
lx : α
lr : Ordnode α
⊢ (casesOn (id nil)
        (casesOn (id (node ls ll lx lr)) (Ordnode.singleton x) fun ls_1 ll_1 lx_1 lr_1 ↦
          casesOn (id ll_1)
            (casesOn lr_1 (node 2 (node ls ll lx lr) x nil) fun size l lrx r ↦
              node 3 (Ordnode.singleton lx_1) lrx (Ordnode.singleton x))
            fun lls l x_1 r ↦
            casesOn (id lr_1) (node 3 ll_1 lx_1 (Ordnode.singleton x)) fun lrs lrl lrx lrr ↦
              if lrs < ratio * lls then node (ls_1 + 1) ll_1 lx_1 (node (lrs + 1) lr_1 x nil)
              else node (ls_1 + 1) (node (lls + lrl.size + 1) ll_1 lx_1 lrl) lrx (node (lrr.size + 1) lrr x nil))
        fun rs l x_1 r ↦
        casesOn (id (node ls ll lx lr)) (node (rs + 1) nil x nil) fun ls_1 ll_1 lx_1 lr_1 ↦
          if ls_1 > delta * rs then
            casesOn (id ll_1) nil fun lls l x_2 r ↦
              casesOn (id lr_1) nil fun lrs lrl lrx lrr ↦
                if lrs < ratio * lls then node (ls_1 + rs + 1) ll_1 lx_1 (node (rs + lrs + 1) lr_1 x nil)
                else
                  node (ls_1 + rs + 1) (node (lls + lrl.size + 1) ll_1 lx_1 lrl) lrx
                    (node (lrr.size + rs + 1) lrr x nil)
          else node (ls_1 + rs + 1) (node ls ll lx lr) x nil).dual =
    casesOn (id nil.dual)
      (casesOn (id (node ls ll lx lr).dual) (Ordnode.singleton x) fun rs rl rx rr ↦
        casesOn (id rr)
          (casesOn rl (node 2 nil x (node ls ll lx lr).dual) fun size l rlx r ↦
            node 3 (Ordnode.singleton x) rlx (Ordnode.singleton rx))
          fun rrs l x_1 r ↦
          casesOn (id rl) (node 3 (Ordnode.singleton x) rx rr) fun rls rll rlx rlr ↦
            if rls < ratio * rrs then node (rs + 1) (node (rls + 1) nil x rl) rx rr
            else node (rs + 1) (node (rll.size + 1) nil x rll) rlx (node (rlr.size + rrs + 1) rlr rx rr))
      fun ls_1 l x_1 r ↦
      casesOn (id (node ls ll lx lr).dual) (node (ls_1 + 1) nil.dual x nil) fun rs rl rx rr ↦
        if rs > delta * ls_1 then
          casesOn (id rr) nil fun rrs l x_2 r ↦
            casesOn (id rl) nil fun rls rll rlx rlr ↦
              if rls < ratio * rrs then node (ls_1 + rs + 1) (node (ls_1 + rls + 1) nil.dual x rl) rx rr
              else
                node (ls_1 + rs + 1) (node (ls_1 + rll.size + 1) nil.dual x rll) rlx
                  (node (rlr.size + rrs + 1) rlr rx rr)
        else node (ls_1 + rs + 1) nil.dual x (node ls ll lx lr).dual

Tactic:
obtain - | ⟨lls, lll, llx, llr⟩ := ll <;> obtain - | ⟨lrs, lrl, lrx, lrr⟩ := lr <;>
      dsimp only [dual, id] <;> try rfl

Goal state after:
case nil.node.node.node
α : Type u_1
x : α
ls : ℕ
lx : α
lls : ℕ
lll : Ordnode α
llx : α
llr : Ordnode α
lrs : ℕ
lrl : Ordnode α
lrx : α
lrr : Ordnode α
⊢ (if lrs < ratio * lls then node (ls + 1) (node lls lll llx llr) lx (node (lrs + 1) (node lrs lrl lrx lrr) x nil)
      else
        node (ls + 1) (node (lls + lrl.size + 1) (node lls lll llx llr) lx lrl) lrx
          (node (lrr.size + 1) lrr x nil)).dual =
    if lrs < ratio * lls then
      node (ls + 1) (node (lrs + 1) nil x (node lrs lrr.dual lrx lrl.dual)) lx (node lls llr.dual llx lll.dual)
    else
      node (ls + 1) (node (lrr.dual.size + 1) nil x lrr.dual) lrx
        (node (lrl.dual.size + lls + 1) lrl.dual lx (node lls llr.dual llx lll.dual))