simp [hx]
case inl
α : Type
inst✝ : BEq α
x a : α
as : List α
hx : (a == x) = true
⊢ (bif a == x then List.countp (fun x_1 => x_1 == x) as + (0 + 1) else List.countp.go (fun x_1 => x_1 == x) as 0) =
    (bif a == x then 1 else 0) + List.countp (fun x_1 => x_1 == x) as
case inl
α : Type
inst✝ : BEq α
x a : α
as : List α
hx : (a == x) = true
⊢ List.countp (fun x_1 => x_1 == x) as + 1 = 1 + List.countp (fun x_1 => x_1 == x) as