static void AddOrUpdateInstructor(string courseTitle, string instructorName)
{
var course = Courses.SingleOrDefault(c => c.Title == courseTitle);
var inst = course.Instructors.SingleOrDefault(i => i.LastName == instructorName);
if (inst == null)
{
var instructor = Instructors.Single(i => i.LastName == instructorName);
course.Instructors.Add(instructor);
}
}
