Prairie Restaurant is a famous old restaurant on the prairie, but it has some unwritten rules , if prairie restaurant sells both braised mutton and haggis soup on a certain day, it must also sell roast whole lamb; The restaurant never sells roast whole lamb on Sundays; ... People are familiar with these rules and take them for granted. In addition, we also know that Boss Wang only goes to eat at the Grassland Restaurant when he sells braised mutton.
If this assertion is true, then it must also be true that the Prairie Restaurant doesn't sell haggis soup on Sundays.