Pragmatic Sensitivity in NL Interfaces and the Structure of Conversation

We t h e r e f o r e propose three ru les t h a t de f ine the no t ion of we l l fo rmed conversa t ion in the present c o n t e x t , making the f o l l o w i n g assumptions. The s t r u c t u r e of a conversat ion can be represented as a t r e e s t r u c t u r e . The wel l formedness of such t rees can be def ined by r e w r i t e r u l e s . The maximal number of l eve l s of embedding in such t rees i s s i x (see below). In p a r t i c u l a r , subdialogues can be embedded w i t h i n d ia logues, but there can be no embedding w i t h i n subdialogues. The l a s t r e s t r i c t i o n c o n f l i c t s w i th what people do. It i s one of the r e s t r i c t i o n s we consider necessary, and which can be handled in such a way t h a t the user w i l l not no t i ce 36 t h a t any such r e s t r i c t i o n e x i s t s . We assume t h a t the f o l l o w i n g four ca tego r ies are s u f f i c i e n t f o r the r e p r e s e n t a t i o n of the s t r u c t u r e of conversa t ion . The symbols used serve as mnemonics f o r t h e i r approximate coun te rpa r t s in Eng l ish , but they should not be s t r i c t l y equated with them: CONV ( conve rsa t i on ) , DIAL (d i a l ogue ) , EXCH (exchange) and MOVE (as discussed above). To fo rmula te in in fo rmal terms the general s t y l e and atmosphere of the r u l es t h a t we w i l l propose more f o r m a l l y below, l e t us say t h a t a CONV may cons i s t of one or more DIALs, a DIAL may cons i s t of one or more EXCHs, and an EXCH cons i s t s of two MOVEs, wi th each of these MOVEs fo l lowed by an op t i ona l DIAL. A major po in t about conversat ions t h a t must be handled in a grammar of t h i s type is the f a c t t h a t al though MOVEs are the only te rmina l nodes, and are t h e r e f o r e the nodes t h a t correspond to the u t te rances t h a t are a c t u a l l y produced, wi th a l l o ther nodes represent ing more a b s t r a c t elements, c e r t a i n f ea tu res of conversa t ion need to be assoc ia ted wi th these a b s t r a c t nodes. For example, al though each MOVE is s p e c i f i e d f o r who the cu r ren t user of the system is and each MOVE also has a p a r t i c u l a r t o p i c , as discussed above, these not ions p roper l y belong to more a b s t r a c t l eve ls of conversa t iona l s t r u c t u r e . Who the u s e r i s can be def ined at the CONV leve l ( i . e . we de f ine a CONV as a conversat ion wi th one use r ) . The t o p i c of an u t te rance can be def ined a t the DIAL leve l ( i . e . a CONV can cons i s t of one or more d ia logues, each on a s ing le t o p i c ) . Furthermore, a DIAL can cons i s t of one or more EXCHs, and i t i s a t t h i s po in t t h a t the content of the u t te rances t h a t form par t of t h a t EXCH is de f ined . Let us now be more p rec ise . We assume t h a t some of the s l o t s mentioned above in the MOVE frame are represented as f ea tu res on the nodes in the t rees represen t ing the s t r u c t u r e of the conversa t ion of which the moves descr ibed by the MOVE frames are p a r t . This assoc ia t i on of f ea tu res wi th nodes, plus the assumption t h a t a l l f ea tu res t r i c k l e down, wi th a few except ions discussed below, prov ides f o r t r ees of the general form shown in Table i . The lower case l e t t e r s are constants . Note t h a t the values of the BASE fea tu re on the subdialogue nodes have not been s p e c i f i e d . We r e t u r n to t h i s po in t below. Table I represents a goa l : the s o r t of s t r u c t u r e s we want the r u l es to produce. The f o l l o w i n g th ree r u l es generate t r ees of t h i s type. Kleene Plus n o t a t i o n is used. The n o t a t i o n should be i n t e r p r e t e d as f o l l o w s . Roman l e t t e r s as f ea tu re values are constants . Greek l e t t e r s are v a r i a b l e s ranging over poss ib le f ea tu re va lues, and are to be i n t e r p r e t e d c o n s i s t e n t l y w i t h i n a rule~ but not necessa r i l y between r u l e s . They are used to ensure t h a t the c o r r e c t