congr
t : ℕ
P : ℕ → ℕ
h : Balanced t P
f : ℕ → ℕ
⊢ ∑ x in smallParts h, f (P x) + ∑ x in largeParts h, f (P x) =
    card (smallParts h) * f (minP t P) + card (largeParts h) * f (minP t P + 1)
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 largeParts h, f (P x) = card (largeParts h) * f (minP t P + 1)