This two-star logic puzzle has 4 noun types, 5 nouns per type, 2 links, 20 facts, and 1 rule that has only a trigger. Since each student returns a book on either Wednesday or Thursday, the days are Wednesday1, Wednesday2, Wednesday3, Thursday1, and Thursday2. To avoid multiple solutions, based on clue 1 the facts have Mary returning her book on Wednesday1, Ms Snell on Wednesday2, and the Hemingway novel on Wednesday3. And based on clue 5, the facts have Barr returning the book on Thursday1, and the Conrad novel on Thursday2.
Fact 9 states "Wicks and Jones returned their books on the same day." If you disable the rule before solving the puzzle, and enable only Level 1, the program will find 92/150 marks and 7/30 pairs. Looking at the grid for days and last names, Wicks can be Wednesday1, Wednesday3, and Thursday2, while Jones can be Wednesday1, Wednesday3, and Thursday2. Wednesday is not a problem, but Thursday is not possible.
If we enable Level 2, the program will enter mark 93 'X' for Thursday2 and Wicks. Level 2b examines fact 9 and determines that if Wicks has Thursday2, then Jones cannot be on the same day as Wicks. The program finds a solution as well.
If we enable the rule, the program will invoke it to trigger mark 53: 'X' for Thursday2 and Jones. With this rule, the program can find the solution using only Level 1a.