This is server API (go to client)
Module Eliom_react.S
module S : sig..end
create param creates an Up event.
If ~name is present, the coservice used to transmit the event will
always have the same name, even if the server is restarted.
~scope describes the visibility of the event. By default, it is
`Global if it is called during initialisation,
`Client_process otherwise.
module Down : sig..end
