show ((elems α).map Sum.inl ++ (elems β).map Sum.inr).length = _
α : Type u
β : Type v
inst✝¹ : BishopFin α
inst✝ : BishopFin β
⊢ card (α ⊕ β) = card α + card β
α : Type u
β : Type v
inst✝¹ : BishopFin α
inst✝ : BishopFin β
⊢ List.length (List.map Sum.inl (elems α) ++ List.map Sum.inr (elems β)) = card α + card β