First of all, the criteria are described that are used to identify the elementary units of undering structure and the operations conoining them into complex units (), t h e n t h e m a i n t y p e s o f ~ n ~ t s and o p e r a t i o n s resulting from an empirical investigation on t h e b a s i s o f t h e c r i t e r i a are registered ( S e c t . 2 ) ,.