type Stream[T] = concept # interface
  func head(_: Self): T
  func tail(_: Self): Self

proc display[T](stream: Stream[T]) =
  stdout.write stream.head
  stream.tail.display

func consumes[T: char](a: Stream[T]) = discard

func head[T](stream: seq[T]): T = stream.data[0]

func tail[T](stream: seq[T]): seq[T] = stream


let a = @['h', 'e', 'l', 'l', 'o']


consumes(a)