(* an infinite list of values with type 'a *)
type 'a stream

(* const v is an infinite list of values v *)
val const : 'a -> 'a stream

(* return the first element and the stream that follows it *)
val next : 'a stream -> 'a * 'a stream

(* cons x f is the stream with x followed by f() *)
val cons : 'a -> (unit -> 'a stream) -> 'a stream

(* filter pred s:                                               *
 * return a new stream s' containing only those elements v of s *
 * such that pred v == true                                     *)
val filter : ('a -> bool) -> 'a stream -> 'a stream

(* map f s:                                      *
 * apply f to all elements of an infinite stream *)
val map : ('a -> 'b) -> 'a stream -> 'b stream

(* scan f b s: 
 * if s is the stream v0; v1; v2; ... then return the stream    *
 * f b v0; f (f b v0) v1; f (f (f b v0) v1) v2; ...             *)
val scan : ('b -> 'a -> 'b) -> 'b -> 'a stream -> 'b stream