The following functions return the current state of the state for a given
scope: Alive_state means that data has been recorded for this scope, Empty_state means that there is no data for this scope, Expired_state means that data for this scope has been removed
because the timeout has been reached.
The default scope is `Session.
This type is used as a phantom type in Eliom_parameter.params_type to describe
whether a parameter is encoded in the path of the URI as a suffix
parameter.