Dllist

`sig
type 'a node_t
exception Empty
val create : 'a -> 'a Dllist.node_t
val copy : 'a Dllist.node_t -> 'a Dllist.node_t
val length : 'a Dllist.node_t -> int
val rev : 'a Dllist.node_t -> unit
val add : 'a Dllist.node_t -> 'a -> unit
val append : 'a Dllist.node_t -> 'a -> 'a Dllist.node_t
val prepend : 'a Dllist.node_t -> 'a -> 'a Dllist.node_t
val promote : 'a Dllist.node_t -> unit
val demote : 'a Dllist.node_t -> unit
val remove : 'a Dllist.node_t -> unit
val drop : 'a Dllist.node_t -> 'a Dllist.node_t
val rev_drop : 'a Dllist.node_t -> 'a Dllist.node_t
val splice : 'a Dllist.node_t -> 'a Dllist.node_t -> unit
val get : 'a Dllist.node_t -> 'a
val set : 'a Dllist.node_t -> 'a -> unit
val next : 'a Dllist.node_t -> 'a Dllist.node_t
val prev : 'a Dllist.node_t -> 'a Dllist.node_t
val skip : 'a Dllist.node_t -> int -> 'a Dllist.node_t
val iter : ('a -> unit) -> 'a Dllist.node_t -> unit
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b Dllist.node_t -> 'a
val fold_right : ('a -> 'b -> 'b) -> 'a Dllist.node_t -> 'b -> 'b
val map : ('a -> 'b) -> 'a Dllist.node_t -> 'b Dllist.node_t
val to_list : 'a Dllist.node_t -> 'a list
val of_list : 'a list -> 'a Dllist.node_t
val enum : 'a Dllist.node_t -> 'a Enum.t
val rev_enum : 'a Dllist.node_t -> 'a Enum.t
val of_enum : 'a Enum.t -> 'a Dllist.node_t
end`