open Vector

datatype t = T of t vector
fun makeT () = T (tabulate (0, fn _ => makeT ()))
fun destT (T v) =
   if length v > 0
      then 1 + destT (sub (v, 0))
   else 0
val _ =
   if 0 = destT (makeT ())
      then ()
   else raise Fail "bug"