This type is used as a phantom type in Eliom_parameter_sigs.S.params_type to
describe whether a parameter is encoded in the path of the URI as
a suffix parameter.
a page can be in one of the following states: Generating: page is currently being generated and not yet instated as
the active page, Active: page is currently being displayed, Cached: page is in the browser history with its DOM stashed in the cache, Dead: page is in the browser history without its DOM being cached