1. http://www.uilots.let.uu.nl/conferences/Perspectives_on_Aspect/Proceedings/stoevsky.pdf.
2. http://www.sear.utoronto.ca/~binnick/TENSE/http.html
3. http://www.let.uu.nl/~Esther.Ruigenjk/personal/pioneer.pdf.
4. http://www.ebbs.english.vt.edu/hel/hel.html