Seq

`sig
type 'a t = unit -> 'a Seq.node
and 'a node = Nil | Cons of 'a * 'a Seq.t
val empty : 'a Seq.t
val return : 'a -> 'a Seq.t
val cons : 'a -> 'a Seq.t -> 'a Seq.t
val append : 'a Seq.t -> 'a Seq.t -> 'a Seq.t
val map : ('a -> 'b) -> 'a Seq.t -> 'b Seq.t
val filter : ('a -> bool) -> 'a Seq.t -> 'a Seq.t
val filter_map : ('a -> 'b option) -> 'a Seq.t -> 'b Seq.t
val flat_map : ('a -> 'b Seq.t) -> 'a Seq.t -> 'b Seq.t
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b Seq.t -> 'a
val iter : ('a -> unit) -> 'a Seq.t -> unit
val unfold : ('b -> ('a * 'b) option) -> 'b -> 'a Seq.t
end`