class empty_fs :string ->stream_fs
This is a class where all methods fail with ENOSYS. The string
      argument is the detail in the Unix_error, normally the module
      name of the user of this class.
empty_fs is intended as base class for implementations of stream_fs
      outside Ocamlnet. When stream_fs is extended by new methods, these
      methods are at least defined, and no build error occurs. So the
      definition should look like
      class my_fs ... =
        object
          inherit Netfs.empty_fs "my_fs"
          method read flags name = ...
          (* Add here all methods you can define, and omit the rest *)
        end