rw [mul_ite_zero]
s : Sieve
d : ℕ
a✝ : d ∈ A
⊢ (if P.gcd d = 1 then s.weights d else 0) = s.weights d * if P.gcd d = 1 then 1 else 0
s : Sieve
d : ℕ
a✝ : d ∈ A
⊢ (if P.gcd d = 1 then s.weights d else 0) = if P.gcd d = 1 then s.weights d * 1 else 0