(* twins.sml *)

(* twins stream transformer *)

structure Twins :> TWINS =
struct

fun twins stm =
      let val init : int * int Stream.stream = Stream.get stm

          fun next (n, stm) =
                let val (m, stm) = Stream.get stm
                in if m = n + 2
                   then ((n, m), (m, stm))
                   else next (m, stm)
                end
      in Stream.make(init, next) end

end;