T.W
type 'a t
val return : 'a -> 'a W.t
type (-'a, 'b) ft
val fmap : ('a, 'b) W.ft -> 'a W.t -> 'b W.t
type 'a tlist
val nil : unit -> 'a W.tlist
val singleton : 'a W.t -> 'a W.tlist
val cons : 'a W.t -> 'a W.tlist -> 'a W.tlist
val append : 'a W.tlist -> 'a W.tlist -> 'a W.tlist
val map : ('a, 'b) W.ft -> 'a W.tlist -> 'b W.tlist