-- Module: Mathlib.Order.JordanHolder
Goal:
X : Type u
inst : Lattice X
inst_1 : JordanHolderLattice X
s : CompositionSeries X
x₁ x₂ y₁ y₂ : X
hsat₁ :
  @IsMaximal _ inst _
    (last (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s)
    x₁
hsat₂ :
  @IsMaximal _ inst _
    (last (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s)
    x₂
hsaty₁ :
  @IsMaximal _ inst _
    ((snoc (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s x₁ hsat₁).last
      (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y}))
    y₁
hsaty₂ :
  @IsMaximal _ inst _
    ((snoc (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s x₂ hsat₂).last
      (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y}))
    y₂
hr₁ :
  @Iso _ inst _
    (last (r :=
        {x : X × X |
          match (motive := X × X → Prop) x with
          | (x, y) => @IsMaximal _ inst _ x y})
        s,
      x₁)
    (x₂, y₂)
hr₂ :
  @Iso _ inst _ (x₁, y₁)
    (last (r :=
        {x : X × X |
          match (motive := X × X → Prop) x with
          | (x, y) => @IsMaximal _ inst _ x y})
        s,
      x₂)
e : Fin
    (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}) +
        1 +
      1) ≃
  Fin
    (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}) +
        1 +
      1) :=
  Equiv.swap
    (Fin.last
      (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}) +
        1))
    (Fin.castSucc
      (Fin.last
        (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}))))
h1 :
  ∀
    {i :
      Fin
        (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}))},
    i.castSucc.castSucc ≠
      Fin.castSucc
        (Fin.last
          (s.length (r :=
            {x : X × X |
              match (motive := X × X → Prop) x with
              | (x, y) => @IsMaximal _ inst _ x y})))
h2 :
  ∀
    {i :
      Fin
        (s.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}))},
    i.castSucc.castSucc ≠
      Fin.last
        (s.length (r :=
            {x : X × X |
              match (motive := X × X → Prop) x with
              | (x, y) => @IsMaximal _ inst _ x y}) +
          1)
i :
  Fin
    (((snoc (r :=
              {x : X × X |
                match (motive := X × X → Prop) x with
                | (x, y) => @IsMaximal _ inst _ x y})
              s x₁ hsat₁).snoc
          (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          y₁ hsaty₁).length
      (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y}))
⊢ @Iso _ inst _ (x₁, y₁)
    (s.toFun (r := {x : X × X | @IsMaximal _ inst _ x.1 x.2})
        (Fin.last (s.length (r := {x : X × X | @IsMaximal _ inst _ x.1 x.2}))),
      x₂)

Tactic:
exact hr₂