1. English Discourse Particles: Evidence from a Corpus;Aijmer,2002
2. Ad hoc categories;Barsalou;Mem. Cognit.,1983
3. Grammar of Spoken and Written English;Biber,1999
4. The Evolution of Grammar: Tense, Aspect, and Modality in the Languages of the World;Bybee,1994
5. Cambridge Grammar of English: A Comprehensive Guide: Spoken and Written English Grammar and Usage;Carter,2006