-- Lean 4 proof step from Mathlib.Topology.Homotopy.HSpaces (goal)
Goal:
X : Type u
Y : Type v
inst : TopologicalSpace X
inst_1 : TopologicalSpace Y
inst_2 : HSpace X
inst_3 : HSpace Y
G : (↑I : Type) × X × Y → X × Y :=
  fun (p : (↑I : Type) × X × Y) ↦
    ((eHmul : (a : (↑I : Type) × X) → X) (p.1, p.2.1), (eHmul : (a : (↑I : Type) × Y) → Y) (p.1, p.2.2))
hG : Continuous G
map_zero_left :
  ∀ (x : X × Y),
    { toFun := G, continuous_toFun := hG : C((↑I : Type) × X × Y, X × Y) }.toFun (0, x) =
      (comp
            {
              toFun := fun (p : (X × Y) × X × Y) ↦
                ((hmul : (a : X × X) → X) (p.1.1, p.2.1), (hmul : (a : Y × Y) → Y) (p.1.2, p.2.2)),
              continuous_toFun := ⋯ : C((X × Y) × X × Y, X × Y) }
            (prodMk (const (X × Y) (@e _ inst _, @e _ inst_1 _)) (ContinuousMap.id (X × Y))) :
          (a : X × Y) → X × Y)
        x
map_one_left :
  ∀ (x : X × Y),
    { toFun := G, continuous_toFun := hG : C((↑I : Type) × X × Y, X × Y) }.toFun (1, x) =
      (ContinuousMap.id (X × Y) : (a : X × Y) → X × Y) x
⊢ HomotopyRel
    (comp
      {
        toFun := fun (p : (X × Y) × X × Y) ↦
          ((hmul : (a : X × X) → X) (p.1.1, p.2.1), (hmul : (a : Y × Y) → Y) (p.1.2, p.2.2)),
        continuous_toFun := ⋯ : C((X × Y) × X × Y, X × Y) }
      (prodMk (const (X × Y) (@e _ inst _, @e _ inst_1 _)) (ContinuousMap.id (X × Y))))
    (ContinuousMap.id (X × Y)) {(@e _ inst _, @e _ inst_1 _)}
have prop' : ∀ (t : (↑I : Type)) (x : X × Y),
  Membership.mem (γ := Set (X × Y)) {(@e _ inst _, @e _ inst_1 _)} x →
    Eq (α := X × Y)
      (({
              toFun := fun (x : X × Y) ↦
                (Homotopy.toContinuousMap (f₀ :=
                      comp
                        {
                          toFun := fun (p : (X × Y) × X × Y) ↦
                            ((hmul : (a : X × X) → X) (p.1.1, p.2.1), (hmul : (a : Y × Y) → Y) (p.1.2, p.2.2)),
                          continuous_toFun := sorry : C((X × Y) × X × Y, X × Y) }
                        (prodMk (const (X × Y) (@e _ inst _, @e _ inst_1 _)) (ContinuousMap.id (X × Y))))
                      (f₁ := ContinuousMap.id (X × Y))
                      { toFun := G, continuous_toFun := hG, map_zero_left := sorry, map_one_left := sorry :
                        Homotopy
                          (comp
                            {
                              toFun := fun (p : (X × Y) × X × Y) ↦
                                ((hmul : (a : X × X) → X) (p.1.1, p.2.1), (hmul : (a : Y × Y) → Y) (p.1.2, p.2.2)),
                              continuous_toFun := sorry : C((X × Y) × X × Y, X × Y) }
                            (prodMk (const (X × Y) (@e _ inst _, @e _ inst_1 _)) (ContinuousMap.id (X × Y))))
                          (ContinuousMap.id (X × Y)) }).toFun
                  (t, x),
              continuous_toFun := sorry : C(X × Y, X × Y) } :
            (a : X × Y) → X × Y)
          x :
        X × Y)
      ((comp
              {
                toFun := fun (p : (X × Y) × X × Y) ↦
                  ((hmul : (a : X × X) → X) (p.1.1, p.2.1), (hmul : (a : Y × Y) → Y) (p.1.2, p.2.2)),
                continuous_toFun := sorry : C((X × Y) × X × Y, X × Y) }
              (prodMk (const (X × Y) (@e _ inst _, @e _ inst_1 _)) (ContinuousMap.id (X × Y))) :
            (a : X × Y) → X × Y)
          x :
        X × Y)
Tactic: use! ⟨G, hG⟩