My objective is to give a general description of the levels in the Mystery Master application. In games such as Ms. Pac-Man, you must successfully complete a level before you can play the next one. Mystery Master attempts the same thing, albeit without the raw thrills of the chase...
Mystery Master invokes each level to find marks. Only when a level is complete will it move on to the next level. While levels are used to find marks, laws are invoked on the mark to (1) validate the mark, and (2) find additional marks based on the current mark. Before you continue, please read the article on facts.
The examples I will use are taken from the logic puzzle "Five Houses". The levels are given below.
Level 1 examines one fact at a time to enter one or more marks. This level has several parts to it. In essence, it starts with facts of type 1 (where the link is "with"), moves on to facts of type 2, then examines facts of type 3 and 4. Type 1 facts always yield exactly one mark, while the other types of facts usually yield more than one. This puzzle, along with most puzzles, does not have type 2 facts.
Below are the facts of type 3 or 4.
Level 2 examines two facts at a time to enter one or more marks. This level has several parts as well. In general, it looks at two facts at a time where the links for both facts have the same noun type. Logic puzzles that benefit from this level include: "Circus Anniversaries", "Fancy Footwear", "Hello Dolly!", "Mixed-Up Mythology", "The Seven Wives of Henri VII", "Six Fearsome Heroes", and "Witnesses".
Level 3 examines all facts at a time to enter one or more marks. This level is for difficult puzzles such as "Hello Dolly!" and "The Seven Wives of Henri VII".
Level 4 is the level of last resort. If a logic puzzle cannot be solved with its facts and rules, then assumptions must be made. Usually assumptions are necessary for difficult puzzles. If the program makes an assumption, then later finds a contradiction or violation, the program must undo the assumption, and make another one. Even when the program finds a solution, it will undo the last assumption it made to see if there is another solution. The program will proceed in this manner until all possibilities have been exhausted. If there are multiple solutions, this usually means the puzzle was not properly designed and/or implemented.
For more information, please read the article on assumptions.
Levels are invoked to find marks in a logical and systematic manner. You could learn a lot about solving logic puzzles by understanding how each level works.