rw [card_eq_sum_ones, sum_mul, one_mul]
case e_a
t : ℕ
P : ℕ → ℕ
h : Balanced t P
f : ℕ → ℕ
⊢ ∑ x in smallParts h, f (P x) = card (smallParts h) * f (minP t P)
case e_a
t : ℕ
P : ℕ → ℕ
h : Balanced t P
f : ℕ → ℕ
⊢ ∑ x in smallParts h, f (P x) = ∑ x in smallParts h, f (minP t P)