conv => {lhs; congr; {skip}; {ext i; rw [← ite_zero_mul]}}
s : SelbergSieve
⊢ (∑ l ∈ P.divisors, if ↑l ^ 2 ≤ y then g l * S⁻¹ ^ 2 else 0) = S⁻¹
s : SelbergSieve
⊢ ∑ i ∈ P.divisors, (if ↑i ^ 2 ≤ y then g i else 0) * S⁻¹ ^ 2 = S⁻¹