Conex_unix_private_keySourcePrivate key IO on Unix
include Conex_private.FSids () is the list of available identifiers.
val read :
(float -> Conex_resource.timestamp option) ->
Conex_resource.identifier ->
(string * Conex_resource.timestamp, string) resultread id is either the content and creation timestamp of id, or an error.
write id data stores data as id persistently, or errors.