-- Lean 4 proof step from Mathlib.CategoryTheory.SmallObject.Construction (goal)
Goal:
C : Type u
inst : Category.{v, u} C
I : Type w
A B : I → C
f : (i : I) → A i ⟶ B i
S X : C
πX : X ⟶ S
inst_1 : HasColimitsOfShape (Discrete (FunctorObjIndex f πX)) C
inst_2 :
  HasPushout (X := ∐ functorObjSrcFamily f πX) (Z := ∐ functorObjTgtFamily f πX)
    (functorObjTop f πX : ∐ functorObjSrcFamily f πX ⟶ X)
    (functorObjLeft f πX : ∐ functorObjSrcFamily f πX ⟶ ∐ functorObjTgtFamily f πX)
inst_3 : HasPushouts.{v, u} C
inst_4 : ∀ {X S : C} (πX : X ⟶ S), HasColimitsOfShape (Discrete (FunctorObjIndex f πX)) C
π₁ π₂ π₃ : Arrow.{v, u} C
τ : π₁ ⟶ π₂
τ' : π₂ ⟶ π₃
⊢ Eq (α := functorObj f π₁.hom ⟶ functorObj f π₃.hom) (functorMap f (τ ≫ τ'))
    (CategoryStruct.comp (Y := functorObj f π₂.hom) (functorMap f τ) (functorMap f τ'))
have h₁ : Eq (α := functorObj f π₁.hom ⟶ functorObj f π₃.hom)
  (pushout.map (functorObjTop f π₁.hom) (functorObjLeft f π₁.hom) (functorObjTop f π₃.hom) (functorObjLeft f π₃.hom)
      (Arrow.Hom.left τ ≫ Arrow.Hom.left τ') (functorMapTgt f (τ ≫ τ')) (functorMapSrc f (τ ≫ τ')) sorry sorry :
    pushout (X := ∐ functorObjSrcFamily f π₁.hom) (Z := ∐ functorObjTgtFamily f π₁.hom)
        (functorObjTop f π₁.hom : ∐ functorObjSrcFamily f π₁.hom ⟶ π₁.left)
        (functorObjLeft f π₁.hom : ∐ functorObjSrcFamily f π₁.hom ⟶ ∐ functorObjTgtFamily f π₁.hom) ⟶
      pushout (X := ∐ functorObjSrcFamily f π₃.hom) (Z := ∐ functorObjTgtFamily f π₃.hom)
        (functorObjTop f π₃.hom : ∐ functorObjSrcFamily f π₃.hom ⟶ π₃.left)
        (functorObjLeft f π₃.hom : ∐ functorObjSrcFamily f π₃.hom ⟶ ∐ functorObjTgtFamily f π₃.hom))
  (CategoryStruct.comp (Y := functorObj f π₂.hom)
      (pushout.map (functorObjTop f π₁.hom) (functorObjLeft f π₁.hom) (functorObjTop f π₂.hom) (functorObjLeft f π₂.hom)
          (Arrow.Hom.left τ) (functorMapTgt f τ) (functorMapSrc f τ) sorry sorry :
        pushout (X := ∐ functorObjSrcFamily f π₁.hom) (Z := ∐ functorObjTgtFamily f π₁.hom)
            (functorObjTop f π₁.hom : ∐ functorObjSrcFamily f π₁.hom ⟶ π₁.left)
            (functorObjLeft f π₁.hom : ∐ functorObjSrcFamily f π₁.hom ⟶ ∐ functorObjTgtFamily f π₁.hom) ⟶
          pushout (X := ∐ functorObjSrcFamily f π₂.hom) (Z := ∐ functorObjTgtFamily f π₂.hom)
            (functorObjTop f π₂.hom : ∐ functorObjSrcFamily f π₂.hom ⟶ π₂.left)
            (functorObjLeft f π₂.hom : ∐ functorObjSrcFamily f π₂.hom ⟶ ∐ functorObjTgtFamily f π₂.hom))
      (pushout.map (functorObjTop f π₂.hom) (functorObjLeft f π₂.hom) (functorObjTop f π₃.hom) (functorObjLeft f π₃.hom)
          (Arrow.Hom.left τ') (functorMapTgt f τ') (functorMapSrc f τ') sorry sorry :
        pushout (X := ∐ functorObjSrcFamily f π₂.hom) (Z := ∐ functorObjTgtFamily f π₂.hom)
            (functorObjTop f π₂.hom : ∐ functorObjSrcFamily f π₂.hom ⟶ π₂.left)
            (functorObjLeft f π₂.hom : ∐ functorObjSrcFamily f π₂.hom ⟶ ∐ functorObjTgtFamily f π₂.hom) ⟶
          pushout (X := ∐ functorObjSrcFamily f π₃.hom) (Z := ∐ functorObjTgtFamily f π₃.hom)
            (functorObjTop f π₃.hom : ∐ functorObjSrcFamily f π₃.hom ⟶ π₃.left)
            (functorObjLeft f π₃.hom : ∐ functorObjSrcFamily f π₃.hom ⟶ ∐ functorObjTgtFamily f π₃.hom)) :
    functorObj f π₁.hom ⟶ functorObj f π₃.hom)
Tactic: simp only [functorMap, Arrow.comp_left, Arrow.mk_left]