-- Module: Mathlib.Order.JordanHolder
Goal:
X : Type u
inst : Lattice X
inst_1 : JordanHolderLattice X
n : ℕ
ih :
  ∀ (s₁ s₂ : CompositionSeries X)
    (hb :
      head (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s₁ =
        head (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s₂)
    (ht :
      last (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s₁ =
        last (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s₂)
    (hle :
      s₁.length (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y}) =
        n),
    s₁.Equivalent s₂
s₁ s₂ : CompositionSeries X
hb :
  head (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₁ =
    head (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₂
ht :
  last (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₁ =
    last (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₂
hle :
  s₁.length (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y}) =
    n + 1
h0s₂ :
  0 <
    s₂.length (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
t : CompositionSeries X
htb :
  head (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      t =
    head (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₁
htl :
  t.length (r :=
        {x : X × X |
          match (motive := X × X → Prop) x with
          | (x, y) => @IsMaximal _ inst _ x y}) +
      1 =
    s₁.length (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
htt :
  last (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      t =
    (eraseLast (r :=
          {x : X × X |
            match (motive := X × X → Prop) x with
            | (x, y) => @IsMaximal _ inst _ x y})
          s₂).last
      (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
hteq :
  s₁.Equivalent
    (snoc (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      t
      (last (r :=
        {x : X × X |
          match (motive := X × X → Prop) x with
          | (x, y) => @IsMaximal _ inst _ x y})
        s₁)
      ⋯)
⊢ t.Equivalent
    (eraseLast (r :=
      {x : X × X |
        match (motive := X × X → Prop) x with
        | (x, y) => @IsMaximal _ inst _ x y})
      s₂)

Tactic:
have := ih t s₂.eraseLast (by simp [htb, ← hb]) htt (Nat.succ_inj.1 (htl.trans hle))