Your TAs are helping the students to form homework groups, so they have every student fill out a form listing all of the other students who they would be willing to work with. There are 251 students in the class, and every student lists exactly 168 other students who they would be willing to work with. For any two students in the class, if student A puts student B on their list, then student B will also have student A on their list. Using the Pigeonhole Principle, show that there must be some group of 4 students who are all willing to work with one another.