inductive HList {α : Type v} (β : α → Type u) : List α → Type (max u v)
  | nil  : HList β []
  | cons : β i → HList β is → HList β (i::is)

infix:67 " :: " => HList.cons

inductive Member : α → List α → Type
  | head : Member a (a::as)
  | tail : Member a bs → Member a (b::bs)

def HList.get : HList β is → Member i is → β i
  | a::as, .head => a
  | a::as, .tail h => as.get _