Based on the VAIL Tutor.  http://www-apps.umuc.edu/vailtutor.
See also, the Vail tutor bibliography: http://www-apps.umuc.edu/vailtutor/bibliography.html

The Virtual Academic Integrity Laboratory (VAIL) at the University of Maryland University College has granted us explicit permission to use and modify their text.

Modification and additions are copyright Regents of the University of Michigan