-- Mathlib/CategoryTheory/Localization/CalculusOfFractions.lean
/-- The equivalence class of `z₁.comp₀ z₂ z₃` does not depend on the choice of `z₃` provided
they satisfy the compatibility `z₂.f ≫ z₃.s = z₁.s ≫ z₃.f`. -/
lemma comp₀_rel [W.HasLeftCalculusOfFractions]
    {X Y Z : C} (z₁ : W.LeftFraction X Y) (z₂ : W.LeftFraction Y Z)
    (z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y') (h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f)
    (h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f) :
    LeftFractionRel (z₁.comp₀ z₂ z₃) (z₁.comp₀ z₂ z₃')

Proof state:
case intro.intro.intro.intro.refine_3
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y✝ Z : C
z₁ : W.LeftFraction X Y✝
z₂ : W.LeftFraction Y✝ Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
eq : z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s
Y : C
t : z₄.Y' ⟶ Y
ht : W t
fac' : z₃.f ≫ z₄.f ≫ t = z₃'.f ≫ z₄.s ≫ t
⊢ W (z₂.s ≫ z₃'.s ≫ z₄.s ≫ t)

Tactic:
exact W.comp_mem _ _ z₂.hs (W.comp_mem _ _ z₃'.hs (W.comp_mem _ _ z₄.hs ht))

Result:
no goals

Proof state:
case intro.intro.intro.intro.refine_3
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y✝ Z : C
z₁ : W.LeftFraction X Y✝
z₂ : W.LeftFraction Y✝ Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
eq : z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s
Y : C
t : z₄.Y' ⟶ Y
ht : W t
fac' : z₃.f ≫ z₄.f ≫ t = z₃'.f ≫ z₄.s ≫ t
⊢ W ((z₁.comp₀ z₂ z₃).s ≫ z₄.f ≫ t)

Tactic:
simp only [comp₀, assoc, ← reassoc_of% fac]

Result:
case intro.intro.intro.intro.refine_3
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y✝ Z : C
z₁ : W.LeftFraction X Y✝
z₂ : W.LeftFraction Y✝ Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
eq : z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s
Y : C
t : z₄.Y' ⟶ Y
ht : W t
fac' : z₃.f ≫ z₄.f ≫ t = z₃'.f ≫ z₄.s ≫ t
⊢ W (z₂.s ≫ z₃'.s ≫ z₄.s ≫ t)

Proof state:
case intro
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y Z : C
z₁ : W.LeftFraction X Y
z₂ : W.LeftFraction Y Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
eq : z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s
⊢ LeftFractionRel (z₁.comp₀ z₂ z₃) (z₁.comp₀ z₂ z₃')

Tactic:
obtain ⟨Y, t, ht, fac'⟩ := HasLeftCalculusOfFractions.ext _ _ _ z₁.hs eq

Result:
case intro.intro.intro.intro
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y✝ Z : C
z₁ : W.LeftFraction X Y✝
z₂ : W.LeftFraction Y✝ Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
eq : z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s
Y : C
t : z₄.Y' ⟶ Y
ht : W t
fac' : (z₃.f ≫ z₄.f) ≫ t = (z₃'.f ≫ z₄.s) ≫ t
⊢ LeftFractionRel (z₁.comp₀ z₂ z₃) (z₁.comp₀ z₂ z₃')

Proof state:
C : Type u_1
inst✝¹ : Category.{u_3, u_1} C
W : MorphismProperty C
inst✝ : W.HasLeftCalculusOfFractions
X Y Z : C
z₁ : W.LeftFraction X Y
z₂ : W.LeftFraction Y Z
z₃ z₃' : W.LeftFraction z₁.Y' z₂.Y'
h₃ : z₂.f ≫ z₃.s = z₁.s ≫ z₃.f
h₃' : z₂.f ≫ z₃'.s = z₁.s ≫ z₃'.f
z₄ : W.LeftFraction z₃.Y' z₃'.Y'
fac : z₃'.s ≫ z₄.s = z₃.s ≫ z₄.f
⊢ z₁.s ≫ z₃.f ≫ z₄.f = z₁.s ≫ z₃'.f ≫ z₄.s

Tactic:
rw [← reassoc_of% h₃, ← reassoc_of% h₃', fac]

Result:
no goals