-- Lean 4 proof step from Mathlib.CategoryTheory.Join.Basic (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
D : Type u₂
inst_1 : Category.{v₂, u₂} D
a a_1 : C
f : Hom.{v₁, v₂, u₁, u₂} (D := D) (left a) (left a_1)
a_2 : D
g : Hom.{v₁, v₂, u₁, u₂} (left a_1) (right a_2)
a_3 : C
h : Hom.{v₁, v₂, u₁, u₂} (right a_2) (left a_3)
⊢ comp (comp f g) h = comp f (comp g h)
have left : Eq (α := ULift.{v₂, v₁} (a ⟶ a_3))
  (match (motive :=
      (x x_1 x_2 : Join.{v₁, v₂, u₁, u₂} C D) →
        (match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x, x_1 with
          | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
          | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
          | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
          | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)}) →
          (match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x_1, x_2 with
            | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
            | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
            | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
            | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)}) →
            match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x, x_2 with
            | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
            | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
            | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
            | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)})
      left a, right a_2, left a_3, PUnit.unit, h with
    | left _x, left _y, left _z, f, g => { down := f.down ≫ g.down }
    | left _x, left _y, right _z, x, x_1 => PUnit.unit
    | left _x, right _y, right _z, x, x_1 => PUnit.unit
    | right _x, right _y, right _z, f, g => { down := f.down ≫ g.down } :
    match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) left a, left a_3 with
    | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
    | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
    | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
    | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)})
  ({
      down :=
        f.down ≫
          ULift.down.{v₂, v₁}
            (match (motive :=
              (x x_1 x_2 : Join.{v₁, v₂, u₁, u₂} C D) →
                (match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x, x_1 with
                  | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
                  | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
                  | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
                  | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)}) →
                  (match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x_1,
                      x_2 with
                    | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
                    | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
                    | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
                    | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)}) →
                    match (motive := Join.{v₁, v₂, u₁, u₂} C D → Join.{v₁, v₂, u₁, u₂} C D → Type (max v₁ v₂)) x,
                      x_2 with
                    | left x, left y => ULift.{v₂, v₁} (x ⟶ y)
                    | right x, right y => ULift.{v₁, v₂} (x ⟶ y)
                    | left a, right a_4 => PUnit.{max (v₁ + 1) (v₂ + 1)}
                    | right a, left a_4 => PEmpty.{max (v₁ + 1) (v₂ + 1)})
              left a_1, right a_2, left a_3, g, h with
            | left _x, left _y, left _z, f, g => { down := f.down ≫ g.down }
            | left _x, left _y, right _z, x, x_1 => PUnit.unit
            | left _x, right _y, right _z, x, x_1 => PUnit.unit
            | right _x, right _y, right _z, f, g => { down := f.down ≫ g.down }) } :
    ULift.{v₂, v₁} (a ⟶ a_3))
Tactic: simp only [Hom, comp, Category.assoc]