diff --git a/registrar/src/registrar/Main.java b/registrar/src/registrar/Main.java
index fcedc41..347d41d 100644
--- a/registrar/src/registrar/Main.java
+++ b/registrar/src/registrar/Main.java
@@ -9,6 +9,30 @@ class Examples {
         new Cons<Integer>(2, new Cons<Integer>(3, new Mt<Integer>()))
     );
 
+    Teacher singer;
+    Teacher wormwood;
+
+    Course singing;
+    Course arithmetic;
+    Course clobbering;
+
+    Student calvin;
+    Student hobbes;
+    Student susie;
+
+    void init() {
+        singer = new Teacher("Mr. Singer");
+        wormwood = new Teacher("Ms. Wormwood");
+
+        singing = new Course("Singing", singer);
+        arithmetic = new Course("Arithmetic", wormwood);
+        clobbering = new Course("Introduction to Clobbering", wormwood);
+
+        calvin = new Student("Calvin");
+        hobbes = new Student("Hobbes");
+        susie = new Student("Susie Derkins");
+    }
+
     void testILoUniqAppend(Tester t) {
         t.checkExpect(testList.uniqAppend(2), testList);
         t.checkExpect(
@@ -25,6 +49,90 @@ class Examples {
             )
         );
     }
+
+    void testILoHas(Tester t) {
+        t.checkExpect(testList.has(2), true);
+        t.checkExpect(testList.has(6), false);
+    }
+
+    void testILoDropFirst(Tester t) {
+        t.checkExpect(
+            testList.dropFirst(2),
+            new Cons<Integer>(1, new Cons<Integer>(3, new Mt<Integer>()))
+        );
+    }
+
+    void testCourse(Tester t) {
+        init();
+        t.checkExpect(singing.teacher, singer);
+        t.checkExpect(
+            singer.courses,
+            new Cons<Course>(singing, new Mt<Course>())
+        );
+    }
+
+    void testStudentEnroll(Tester t) {
+        init();
+        calvin.enroll(arithmetic);
+        t.checkExpect(
+            calvin.courses,
+            new Cons<Course>(arithmetic, new Mt<Course>())
+        );
+        t.checkExpect(
+            arithmetic.students,
+            new Cons<Student>(calvin, new Mt<Student>())
+        );
+        t.checkExpect(calvin.takesCourse(arithmetic), true);
+        calvin.enroll(arithmetic);
+        t.checkExpect(
+            calvin.courses,
+            new Cons<Course>(arithmetic, new Mt<Course>())
+        );
+        t.checkExpect(
+            arithmetic.students,
+            new Cons<Student>(calvin, new Mt<Student>())
+        );
+
+        singing.setTeacher(wormwood);
+        t.checkExpect(singing.teacher, wormwood);
+        t.checkExpect(
+            wormwood.courses,
+            new Cons<Course>(
+                arithmetic,
+                new Cons<Course>(
+                    clobbering,
+                    new Cons<Course>(singing, new Mt<Course>())
+                )
+            )
+        );
+        t.checkExpect(singer.courses, new Mt<Course>());
+    }
+
+    void testStudentClassmates(Tester t) {
+        init();
+
+        calvin.enroll(singing);
+        hobbes.enroll(singing);
+        susie.enroll(arithmetic);
+
+        t.checkExpect(calvin.classmates(hobbes), true);
+        t.checkExpect(hobbes.classmates(calvin), true);
+        t.checkExpect(calvin.classmates(susie), false);
+        t.checkExpect(susie.classmates(hobbes), false);
+    }
+
+    void testTeacherDejaVu(Tester t) {
+        init();
+
+        calvin.enroll(arithmetic);
+        calvin.enroll(clobbering);
+        hobbes.enroll(clobbering);
+        susie.enroll(singing);
+
+        t.checkExpect(wormwood.dejavu(calvin), true);
+        t.checkExpect(singer.dejavu(susie), false);
+        t.checkExpect(wormwood.dejavu(hobbes), false);
+    }
 }
 
 class School {
@@ -46,7 +154,7 @@ class Course {
     Course(String name, Teacher teacher, ILo<Student> students) {
         this.name = name;
         this.teacher = teacher;
-        this.teacher.assignCourse(this);
+        teacher.assignCourse(this);
         this.students = students;
     }
 
@@ -54,6 +162,29 @@ class Course {
     Course(String name, Teacher teacher) {
         this(name, teacher, new Mt<Student>());
     }
+
+    // Effect: Adds a student.
+    void addStudent(Student student) {
+        // Make sure there's no way to add a student to a course without
+        // being enrolled.
+        if (!student.takesCourse(this)) student.enroll(this);
+        this.students = this.students.uniqAppend(student);
+    }
+
+    // Effect: Sets the teacher.
+    void setTeacher(Teacher newTeacher) {
+        if (!newTeacher.equals(this.teacher)) {
+            this.teacher.removeCourse(this);
+            this.teacher = newTeacher;
+            this.teacher.assignCourse(this);
+        }
+        // If the teacher is the same, do nothing.
+    }
+
+    // Is this Course taught by the Teacher?
+    boolean taughtBy(Teacher teacher) {
+        return this.teacher.equals(teacher);
+    }
 }
 
 class Teacher {
@@ -73,11 +204,25 @@ class Teacher {
 
     // Effect: Assigns the given Course to this Teacher.
     void assignCourse(Course course) {
-        this.courses.uniqAppend(course);
+        if (!course.taughtBy(this)) course.setTeacher(this);
+        this.courses = this.courses.uniqAppend(course);
     }
 
-    // Effect: Enrolls the given Student in this Course.
-    void enroll(Student student) {}
+    // Does this Teacher teach the Course?
+    boolean teaches(Course course) {
+        return this.courses.has(course);
+    }
+
+    // Effect: Removes a Course from this Teacher.
+    // ASSUMES THIS TEACHER WILL BE REPLACED.
+    void removeCourse(Course course) {
+        this.courses = this.courses.dropFirst(course);
+    }
+
+    // Is the Student in one or more of this Teacher's Courses?
+    boolean dejavu(Student student) {
+        return this.courses.moreMap(new StudentTakesCourse(student));
+    }
 }
 
 class Student {
@@ -99,14 +244,53 @@ class Student {
 
     // Effect: Enrolls this student in the given course.
     void enroll(Course course) {
-        course.enroll(this);
-        this.courses.uniqAppend(course);
+        this.courses = this.courses.uniqAppend(course);
+        course.addStudent(this);
+    }
+
+    // Does the student take this course?
+    boolean takesCourse(Course course) {
+        return this.courses.has(course);
+    }
+
+    // Are they in any of the same Courses as this Student?
+    boolean classmates(Student student) {
+        return this.courses.orMap(new StudentTakesCourse(student));
+    }
+}
+
+interface Question<A> {
+    boolean ask(A a);
+}
+
+class StudentTakesCourse implements Question<Course> {
+
+    Student student;
+
+    StudentTakesCourse(Student student) {
+        this.student = student;
+    }
+
+    public boolean ask(Course course) {
+        return this.student.takesCourse(course);
     }
 }
 
 interface ILo<A> {
     // Uniquely append an element (i.e., leaves list unchanged if element is already present.)
     ILo<A> uniqAppend(A a);
+
+    // Drop first instance of element from a list.
+    ILo<A> dropFirst(A a);
+
+    // Does the list have this?
+    boolean has(A a);
+
+    // Do any of the elements satisfy the predicate?
+    boolean orMap(Question<A> question);
+
+    // Do more than one of the elements satisfy the predicate?
+    boolean moreMap(Question<A> question);
 }
 
 class Cons<A> implements ILo<A> {
@@ -123,6 +307,23 @@ class Cons<A> implements ILo<A> {
         if (this.first.equals(a)) return this;
         else return new Cons<A>(this.first, this.rest.uniqAppend(a));
     }
+
+    public ILo<A> dropFirst(A a) {
+        if (this.first.equals(a)) return this.rest;
+        else return new Cons<A>(this.first, this.rest.dropFirst(a));
+    }
+
+    public boolean has(A a) {
+        return this.first.equals(a) || this.rest.has(a);
+    }
+
+    public boolean orMap(Question<A> question) {
+        return question.ask(this.first) || this.rest.orMap(question);
+    }
+
+    public boolean moreMap(Question<A> question) {
+        return question.ask(this.first) && this.rest.orMap(question);
+    }
 }
 
 class Mt<A> implements ILo<A> {
@@ -130,4 +331,20 @@ class Mt<A> implements ILo<A> {
     public ILo<A> uniqAppend(A a) {
         return new Cons<A>(a, this);
     }
+
+    public ILo<A> dropFirst(A a) {
+        throw new Error("Could not find element to drop in list.");
+    }
+
+    public boolean has(A a) {
+        return false;
+    }
+
+    public boolean orMap(Question<A> question) {
+        return false;
+    }
+
+    public boolean moreMap(Question<A> question) {
+        return false;
+    }
 }