`sig
val null : Topkg.fpath
val dash : Topkg.fpath
val exists : Topkg.fpath -> bool Topkg.result
val must_exist : Topkg.fpath -> Topkg.fpath Topkg.result
val delete : ?must_exist:bool -> Topkg.fpath -> unit Topkg.result
val fold :
?skip:(Topkg.fpath -> bool) ->
(Topkg.fpath -> 'a -> 'a) -> 'a -> Topkg.fpath list -> 'a Topkg.result
val read : Topkg.fpath -> string Topkg.result
val write : Topkg.fpath -> string -> unit Topkg.result
val write_subst :
Topkg.fpath -> (string * string) list -> string -> unit Topkg.result
val tmp : unit -> Topkg.fpath Topkg.result
end`