-- Lean 4 proof step from Mathlib.Order.KrullDimension (goal)
Goal:
α : Type u_1
inst : Preorder α
a : α
n : ℕ∞
hne :
  Nonempty.{max 1 (u_1 + 1)}
    { p : LTSeries α //
      RelSeries.last (r :=
          {x : α × α |
            match (motive := α × α → Prop) x with
            | (a, b) => a < b})
          p =
        a }
rw :
  (⨆ (p : LTSeries α),
        ⨆ (_ :
          RelSeries.last (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})
              p =
            a),
          (↑(p.length (r :=
                {x : α × α |
                  match (motive := α × α → Prop) x with
                  | (a, b) => a < b})) :
            ℕ∞)) +
      n =
    ⨆ (p : LTSeries α),
      ⨆ (_ :
        RelSeries.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})
            p =
          a),
        (↑(p.length (r :=
                {x : α × α |
                  match (motive := α × α → Prop) x with
                  | (a, b) => a < b})) :
            ℕ∞) +
          n
rw₁ :
  (⨆ (x :
        { p : LTSeries α //
          RelSeries.last (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})
              p =
            a }),
        (↑((↑x : LTSeries α).length (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})) :
          ℕ∞)) +
      n =
    ⨆ (p : LTSeries α),
      ⨆ (_ :
        RelSeries.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})
            p =
          a),
        (↑(p.length (r :=
                {x : α × α |
                  match (motive := α × α → Prop) x with
                  | (a, b) => a < b})) :
            ℕ∞) +
          n
rw₂ :
  (⨆ (x :
        { p : LTSeries α //
          RelSeries.last (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})
              p =
            a }),
        (↑((↑x : LTSeries α).length (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})) :
          ℕ∞)) +
      n =
    ⨆ (x :
      { p : LTSeries α //
        RelSeries.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})
            p =
          a }),
      (↑((↑x : LTSeries α).length (r :=
              {x : α × α |
                match (motive := α × α → Prop) x with
                | (a, b) => a < b})) :
          ℕ∞) +
        n
⊢ height a + n =
    ⨆ (p : LTSeries α),
      ⨆ (_ :
        RelSeries.last (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})
            p =
          a),
        (↑(p.length (r :=
                {x : α × α |
                  match (motive := α × α → Prop) x with
                  | (a, b) => a < b})) :
            ℕ∞) +
          n
have rw₃ : Eq (α := ℕ∞)
  (⨆ (i :
    { p : LTSeries α //
      RelSeries.last (r :=
          {x : α × α |
            match (motive := α × α → Prop) x with
            | (a, b) => a < b})
          p =
        a }),
    (↑((↑i : LTSeries α).length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})) :
        ℕ∞) +
      n)
  (⨆ (x :
    { p : LTSeries α //
      RelSeries.last (r :=
          {x : α × α |
            match (motive := α × α → Prop) x with
            | (a, b) => a < b})
          p =
        a }),
    (↑((↑x : LTSeries α).length (r :=
            {x : α × α |
              match (motive := α × α → Prop) x with
              | (a, b) => a < b})) :
        ℕ∞) +
      n)
Tactic: rw [height_eq_iSup_last_eq, iSup_subtype', iSup_subtype', ENat.iSup_add]