multiple service menus to a single navigation context fig3 shows an example of an availability schedule the top item is a list of services each line pertains to a particular service each service has an identifier ids next the service has a name the relation between a service and a service menu is stored separately next the service has one or more attributes the pointer to the mass storage mp has not been shown the next item is a list of services menus each line pertains to a particular service menu each service menu has an identifier idsm furthermore each service menu points to a set of one or more services also a few of the relationships r2 have been indicated by appropriate arrows the next item is a list of navigation contexts each line pertains to a particular navigation context each navigation context has an identifier idnc furthermore each navigation context points to a set of one or more services menus also a few of the relationships r1 have been indicated by appropriate arrows furthermore each navigation context has one availability start time given as a date and an instant in hours and minutes other notations and other precisions are applicable as well furthermore each navigation context may have one.