Alternative = <main> Service <alternate> Service extends Service.
Concurrent = <service1> Service <service2> Service extends Service.
TimeOut = <timeout> float <service> Service extends Service.
Url = <url> String extends Service.
Service_List = [ <first> Nonempty_Service_List ] .
WebScript = <service_list> Service_List .
Nonempty_Service_List = <it> Service [ <next> Nonempty_Service_List ] .
Repeat = <service> Service extends Service.
Service : Url | TimeOut | Repeat | Alternative | Concurrent .