apply symm
case pos
s : SelbergSieve
d : ℕ
h_dvd : d ∣ P
⊢ ∑ i ∈ P.divisors, ν d * ((ν d)⁻¹ * g d * ↑(μ d) * S⁻¹ * if (↑d * ↑i) ^ 2 ≤ y ∧ i.Coprime d then g i else 0) =
    ∑ i ∈ P.divisors, S⁻¹ * ↑(μ d) * if d ∣ i ∧ ↑i ^ 2 ≤ y then g i else 0
case pos.a
s : SelbergSieve
d : ℕ
h_dvd : d ∣ P
⊢ (∑ i ∈ P.divisors, S⁻¹ * ↑(μ d) * if d ∣ i ∧ ↑i ^ 2 ≤ y then g i else 0) =
    ∑ i ∈ P.divisors, ν d * ((ν d)⁻¹ * g d * ↑(μ d) * S⁻¹ * if (↑d * ↑i) ^ 2 ≤ y ∧ i.Coprime d then g i else 0)