-- Lean 4 proof step from Mathlib.Order.RelSeries (goal)
Goal:
α : Type u_3
inst : PartialOrder α
inst_1 : BoundedOrder α
inst_2 : WellFoundedLT α
inst_3 : WellFoundedGT α
s : LTSeries α
h₁ :
  RelSeries.head (r :=
      {x : α × α |
        match (motive := α × α → Prop) x with
        | (a, b) => a < b})
      s =
    ⊥
inr :
  ∀
    (this :
      ∀ {α : Type u_3} [inst : PartialOrder α] [inst_4 : BoundedOrder α] [WellFoundedLT α] [WellFoundedGT α]
        (s : LTSeries α)
        (h₁ :
          RelSeries.head (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})
              s =
            ⊥)
        (h₂ :
          RelSeries.last (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})
              s =
            ⊤),
        ∃ (t :
          RelSeries (α := α)
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b})
          (i :
          Fin
              (s.length (r :=
                  {x : α × α |
                    match (motive := α × α → Prop) x with
                    | (a, b) => a < b}) +
                1) ↪
            Fin
              (t.length (r :=
                  {x : α × α |
                    match (motive := α × α → Prop) x with
                    | (a, b) => a ⋖ b}) +
                1)),
          t.toFun (r :=
                  {x : α × α |
                    match (motive := α × α → Prop) x with
                    | (a, b) => a ⋖ b}) ∘
                (⇑i : (a : Fin (s.length + 1)) → Fin (t.length + 1)) =
              s.toFun (r :=
                {x : α × α |
                  match (motive := α × α → Prop) x with
                  | (a, b) => a < b}) ∧
            t.head (r :=
                  {x : α × α |
                    match (motive := α × α → Prop) x with
                    | (a, b) => a ⋖ b}) =
                ⊥ ∧
              t.last (r :=
                  {x : α × α |
                    match (motive := α × α → Prop) x with
                    | (a, b) => a ⋖ b}) =
                ⊤)
    (h₂ :
      ¬RelSeries.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})
            s =
          ⊤),
    ∃ (t :
      RelSeries (α := α)
        {x : α × α |
          match (motive := α × α → Prop) x with
          | (a, b) => a ⋖ b})
      (i :
      Fin
          (s.length (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b}) +
            1) ↪
        Fin
          (t.length (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a ⋖ b}) +
            1)),
      t.toFun (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a ⋖ b}) ∘
            (⇑i : (a : Fin (s.length + 1)) → Fin (t.length + 1)) =
          s.toFun (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b}) ∧
        t.head (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a ⋖ b}) =
            ⊥ ∧
          t.last (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a ⋖ b}) =
            ⊤
⊢ ∃ (t :
    RelSeries (α := α)
      {x : α × α |
        match (motive := α × α → Prop) x with
        | (a, b) => a ⋖ b})
    (i :
    Fin
        (s.length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b}) +
          1) ↪
      Fin
        (t.length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) +
          1)),
    t.toFun (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) ∘
          (⇑i : (a : Fin (s.length + 1)) → Fin (t.length + 1)) =
        s.toFun (r :=
          {x : α × α |
            match (motive := α × α → Prop) x with
            | (a, b) => a < b}) ∧
      t.head (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) =
          ⊥ ∧
        t.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) =
          ⊤
have wlog : ∀ {α : Type u_3} [inst : PartialOrder α] [inst_4 : BoundedOrder α] [WellFoundedLT α] [WellFoundedGT α] (s : LTSeries α)
  (h₁ :
    RelSeries.head (r :=
        {x : α × α |
          match (motive := α × α → Prop) x with
          | (a, b) => a < b})
        s =
      ⊥)
  (h₂ :
    RelSeries.last (r :=
        {x : α × α |
          match (motive := α × α → Prop) x with
          | (a, b) => a < b})
        s =
      ⊤),
  ∃ (t :
    RelSeries (α := α)
      {x : α × α |
        match (motive := α × α → Prop) x with
        | (a, b) => a ⋖ b})
    (i :
    Fin
        (s.length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b}) +
          1) ↪
      Fin
        (t.length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) +
          1)),
    t.toFun (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) ∘
          (⇑i : (a : Fin (s.length + 1)) → Fin (t.length + 1)) =
        s.toFun (r :=
          {x : α × α |
            match (motive := α × α → Prop) x with
            | (a, b) => a < b}) ∧
      t.head (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) =
          ⊥ ∧
        t.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a ⋖ b}) =
          ⊤
Tactic: wlog h₂ : s.last = ⊤
Cleared hypotheses: α, inst, inst_1, inst_2, inst_3, s, h₁