-- Lean 4 proof step from Mathlib.CategoryTheory.Limits.Types.Multicoequalizer (hypothesis)
Goal:
X : Type u
ι : Type w
A : Set X
U : ι → Set X
V : ι → ι → Set X
c : MulticoequalizerDiagram A U V
e : Functor.{w, u, w, u + 1} (WalkingMultispan (MultispanShape.prod ι)) (Type u) :=
  (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan
i₁ i₂ : ι
x : (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).right i₁
x_1 : (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).right i₂
x₁ : X
h₁ : x₁ ∈ U i₁
h₂ : x₁ ∈ U i₂
h :
  Eq.{u + 1} (α :=
    Functor.CoconeTypes.pt.{u, u, w, w} (J := WalkingMultispan (MultispanShape.prod ι)) (F :=
      (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan)
      (((c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan.coconeTypesEquiv.symm :
          (a : Cocone (c.multispanIndex.map Set.functorToTypes).multispan) →
            (c.multispanIndex.map Set.functorToTypes).multispan.CoconeTypes)
        (c.multicofork (x := A) (u := U) (v := V).map Set.functorToTypes)))
    (Functor.CoconeTypes.ι
      (((c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan.coconeTypesEquiv.symm :
          (a : Cocone (c.multispanIndex.map Set.functorToTypes).multispan) →
            (c.multispanIndex.map Set.functorToTypes).multispan.CoconeTypes)
        (c.multicofork (x := A) (u := U) (v := V).map Set.functorToTypes))
      (WalkingMultispan.right i₁)
      (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).right i₁) x₁ h₁))
    (Functor.CoconeTypes.ι
      (((c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan.coconeTypesEquiv.symm :
          (a : Cocone (c.multispanIndex.map Set.functorToTypes).multispan) →
            (c.multispanIndex.map Set.functorToTypes).multispan.CoconeTypes)
        (c.multicofork (x := A) (u := U) (v := V).map Set.functorToTypes))
      (WalkingMultispan.right i₂)
      (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).right i₂) x₁ h₂))
⊢ (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan.ιColimitType
      (WalkingMultispan.right i₁)
      (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).right i₁) x₁ h₁) =
    (c.multispanIndex (x := A) (u := U) (v := V).map Set.functorToTypes).multispan.ιColimitType
      (WalkingMultispan.right i₂)
      (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).right i₂) x₁ h₂)
have eq₁ : e.ιColimitType (WalkingMultispan.right ((MultispanShape.prod ι).fst (i₁, i₂)))
    ((ConcreteCategory.hom (FC := TypeCat.Fun) (e.map (WalkingMultispan.Hom.fst (i₁, i₂))) :
        (a : (fun (X : Type u) ↦ X) (e.obj (WalkingMultispan.left (i₁, i₂)))) →
          (fun (X : Type u) ↦ X) (e.obj (WalkingMultispan.right ((MultispanShape.prod ι).fst (i₁, i₂)))))
      (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).left (i₁, i₂)) x₁ sorry)) =
  e.ιColimitType (WalkingMultispan.left (i₁, i₂))
    (Subtype.mk (p := fun (x : X) ↦ x ∈ c.multispanIndex (x := A) (u := U) (v := V).left (i₁, i₂)) x₁ sorry)
Tactic: have eq₁ := e.ιColimitType_map (WalkingMultispan.Hom.fst (J := .prod ι) ⟨i₁, i₂⟩)
      ⟨x₁, by dsimp; rw [c.eq_inf]; exact ⟨h₁, h₂⟩⟩