Module: Mathlib.AlgebraicGeometry.ProjectiveSpectrum.Scheme

Goal state before:
pp: A : Type u_1
σ : Type u_2
inst✝³ : CommRing A
inst✝² : SetLike σ A
inst✝¹ : AddSubgroupClass σ A
𝒜 : ℕ → σ
inst✝ : GradedRing 𝒜
f : A
m : ℕ
f_deg : f ∈ 𝒜 m
q : ↑↑(Spec A⁰_ f).toPresheafedSpace
a b : A
ha : a ∈ carrier f_deg q
hb : b ∈ carrier f_deg q
i : ℕ
⊢ HomogeneousLocalization.mk
      { deg := m * i + m * i,
        num := ⟨∑ x ∈ range (m + m + 1), (m + m).choose x • ((proj 𝒜 i) a ^ x * (proj 𝒜 i) b ^ (m + m - x)), ⋯⟩,
        den := ⟨f ^ (i + i), ⋯⟩, den_mem := ⋯ } ∈
    q.asIdeal

Tactic:
let g : ℕ → A⁰_ f := fun j => (m + m).choose j •
      if h2 : m + m < j then (0 : A⁰_ f)
      else
        if h1 : j ≤ m then
          (HomogeneousLocalization.mk
            ⟨m * i, ⟨proj 𝒜 i a ^ j * proj 𝒜 i b ^ (m - j), ?_⟩,
              ⟨_, by rw [mul_comm]; mem_tac⟩, ⟨i, rfl⟩⟩ : A⁰_ f) *
          (HomogeneousLocalization.mk
            ⟨m * i, ⟨proj 𝒜 i b ^ m, by rw [← smul_eq_mul]; mem_tac⟩,
              ⟨_, by rw [mul_comm]; mem_tac⟩, ⟨i, rfl⟩⟩ : A⁰_ f)
        else
          (HomogeneousLocalization.mk
            ⟨m * i, ⟨proj 𝒜 i a ^ m, by rw [← smul_eq_mul]; mem_tac⟩,
              ⟨_, by rw [mul_comm]; mem_tac⟩, ⟨i, rfl⟩⟩ : A⁰_ f) *
          (HomogeneousLocalization.mk
            ⟨m * i, ⟨proj 𝒜 i a ^ (j - m) * proj 𝒜 i b ^ (m + m - j), ?_⟩,
              ⟨_, by rw [mul_comm]; mem_tac⟩, ⟨i, rfl⟩⟩ : A⁰_ f)

Goal state after:
case refine_3
A : Type u_1
σ : Type u_2
inst✝³ : CommRing A
inst✝² : SetLike σ A
inst✝¹ : AddSubgroupClass σ A
𝒜 : ℕ → σ
inst✝ : GradedRing 𝒜
f : A
m : ℕ
f_deg : f ∈ 𝒜 m
q : ↑↑(Spec A⁰_ f).toPresheafedSpace
a b : A
ha : a ∈ carrier f_deg q
hb : b ∈ carrier f_deg q
i : ℕ
g : ℕ → A⁰_ f :=
  fun j ↦
    (m + m).choose j •
      if h2 : m + m < j then 0
      else
        if h1 : j ≤ m then
          HomogeneousLocalization.mk
              { deg := m * i, num := ⟨(proj 𝒜 i) a ^ j * (proj 𝒜 i) b ^ (m - j), ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ } *
            HomogeneousLocalization.mk { deg := m * i, num := ⟨(proj 𝒜 i) b ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }
        else
          HomogeneousLocalization.mk { deg := m * i, num := ⟨(proj 𝒜 i) a ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ } *
            HomogeneousLocalization.mk
              { deg := m * i, num := ⟨(proj 𝒜 i) a ^ (j - m) * (proj 𝒜 i) b ^ (m + m - j), ⋯⟩, den := ⟨f ^ i, ⋯⟩,
                den_mem := ⋯ }
⊢ HomogeneousLocalization.mk
      { deg := m * i + m * i,
        num := ⟨∑ x ∈ range (m + m + 1), (m + m).choose x • ((proj 𝒜 i) a ^ x * (proj 𝒜 i) b ^ (m + m - x)), ⋯⟩,
        den := ⟨f ^ (i + i), ⋯⟩, den_mem := ⋯ } ∈
    q.asIdeal

case refine_1
A : Type u_1
σ : Type u_2
inst✝³ : CommRing A
inst✝² : SetLike σ A
inst✝¹ : AddSubgroupClass σ A
𝒜 : ℕ → σ
inst✝ : GradedRing 𝒜
f : A
m : ℕ
f_deg : f ∈ 𝒜 m
q : ↑↑(Spec A⁰_ f).toPresheafedSpace
a b : A
ha : a ∈ carrier f_deg q
hb : b ∈ carrier f_deg q
i j : ℕ
h2 : ¬m + m < j
h1 : j ≤ m
⊢ (proj 𝒜 i) a ^ j * (proj 𝒜 i) b ^ (m - j) ∈ 𝒜 (m * i)

case refine_2
A : Type u_1
σ : Type u_2
inst✝³ : CommRing A
inst✝² : SetLike σ A
inst✝¹ : AddSubgroupClass σ A
𝒜 : ℕ → σ
inst✝ : GradedRing 𝒜
f : A
m : ℕ
f_deg : f ∈ 𝒜 m
q : ↑↑(Spec A⁰_ f).toPresheafedSpace
a b : A
ha : a ∈ carrier f_deg q
hb : b ∈ carrier f_deg q
i j : ℕ
h2 : ¬m + m < j
h1 : ¬j ≤ m
⊢ (proj 𝒜 i) a ^ (j - m) * (proj 𝒜 i) b ^ (m + m - j) ∈ 𝒜 (m * i)