The Music Library folder contains a collection of sample music that can be examined with Counterpointer to gain more understanding of different styles of polyphony. You'll find here a number of Bach works, including chorales and fugues, some some from earlier eras and schools, such as Palestrina (the 16th century "gold standard" for counterpoint study) some pieces from the English school, and even one brief anonymous Kyrie from the 15th century (San Pietro BN 80).
You can use the Open command in the File Menu to open any piece in the local library, and you can try evaluating them using various combinations of style rules. You'll find that few composers are entirely consistent in their use of the rules, but the manner of their stylistic departures is instructive. For example, the accented non-suspension dissonance is not typical of the Palestrina style, but when he does employ that figure it is usually a fourth with the bass, which is only technically dissonant, and/or a very quick note approached and left stepwise.
We have also found on several occasions that what appeared to be a stylistic departure identified by Counterpointer actually was just an error in the transcription of the piece - a missing tie, or a mispelled accidental. Checking a work this way is particularly helpful if you are importing music from a MIDI file, since MIDI files have no way to identify specific accidentals.
You also have access to Ars Nova's complete online library via the file menu command "Open online music at Ars Nova."