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datatype 'a seq = Nil
                | Cons of 'a * (unit -> 'a seq)

fun hd (Cons(x, _)) = x
  | hd Nil          = raise Empty

fun tl (Cons(_, xf)) = xf()
  | tl Nil           = raise Empty

fun null Nil = true
  | null _   = false

fun cons (x, xq) = Cons(x, fn () => xq)