(** {2 Filesystem} *)
val is_dir: string -> bool (** @return true if file is a directory *)
+val writable_dir: string -> bool (** @return true if the directory is writable *)
val is_regular: string -> bool (** @return true if file is a regular file *)
val mkdir: string -> unit (** create dir and parents. @raise Failure *)
val tilde_expand: string -> string (** bash-like (head) tilde expansion *)