-- Mathlib/AlgebraicGeometry/ProjectiveSpectrum/Scheme.lean
theorem carrier.add_mem (q : Spec.T A⁰_ f) {a b : A} (ha : a ∈ carrier f_deg q)
    (hb : b ∈ carrier f_deg q) : a + b ∈ carrier f_deg q

Proof state:
case neg
R : Type u_1
A : Type u_2
inst✝³ : CommRing R
inst✝² : CommRing A
inst✝¹ : Algebra R A
𝒜 : ℕ → Submodule R A
inst✝ : GradedAlgebra 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 := ⋯ }
j : ℕ
hj : j ∈ range (m + m + 1)
h2 : ¬m + m < j
h1 : ¬j ≤ m
⊢ Localization.mk ((m + m).choose j • ((proj 𝒜 i) a ^ j * (proj 𝒜 i) b ^ (m + m - j))) ⟨f ^ (i + i), ⋯⟩ =
    (m + m).choose j •
      HomogeneousLocalization.val
        (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 := ⋯ })

Tactic:
simp only [HomogeneousLocalization.val_mul, HomogeneousLocalization.val_zero,
HomogeneousLocalization.val_mk, Subtype.coe_mk, Localization.mk_mul, ← smul_mk]

Result:
case neg
R : Type u_1
A : Type u_2
inst✝³ : CommRing R
inst✝² : CommRing A
inst✝¹ : Algebra R A
𝒜 : ℕ → Submodule R A
inst✝ : GradedAlgebra 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 := ⋯ }
j : ℕ
hj : j ∈ range (m + m + 1)
h2 : ¬m + m < j
h1 : ¬j ≤ m
⊢ (m + m).choose j • Localization.mk ((proj 𝒜 i) a ^ j * (proj 𝒜 i) b ^ (m + m - j)) ⟨f ^ (i + i), ⋯⟩ =
    (m + m).choose j •
      Localization.mk ((proj 𝒜 i) a ^ m * ((proj 𝒜 i) a ^ (j - m) * (proj 𝒜 i) b ^ (m + m - j)))
        (⟨f ^ i, ⋯⟩ * ⟨f ^ i, ⋯⟩)

Proof state:
R : Type u_1
A : Type u_2
inst✝³ : CommRing R
inst✝² : CommRing A
inst✝¹ : Algebra R A
𝒜 : ℕ → Submodule R A
inst✝ : GradedAlgebra 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 :=
          { deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.deg +
            { deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.deg,
        num :=
          ⟨↑{ deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.num *
              ↑{ deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.num,
            ⋯⟩,
        den :=
          ⟨↑{ deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.den *
              ↑{ deg := m * i, num := ⟨(proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i, ⋯⟩, den_mem := ⋯ }.den,
            ⋯⟩,
        den_mem := ⋯ } ∈
    q.asIdeal

Tactic:
dsimp only [Subtype.coe_mk]

Result:
R : Type u_1
A : Type u_2
inst✝³ : CommRing R
inst✝² : CommRing A
inst✝¹ : Algebra R A
𝒜 : ℕ → Submodule R A
inst✝ : GradedAlgebra 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 := ⟨(proj 𝒜 i) (a + b) ^ m * (proj 𝒜 i) (a + b) ^ m, ⋯⟩, den := ⟨f ^ i * f ^ i, ⋯⟩,
        den_mem := ⋯ } ∈
    q.asIdeal