The user teaching_assistant is a member of the group students.
As the teaching assistant for a course, this user should have the ability to write and modify all files in the folder Lecture_Notes.
Other students should be able to read all of these files, but not write or modify them.
This was set up correctly at the beginning of the course. But now, for some reason, teaching_assistant is currently unable to modify the file Lecture4.txt. Find and fix the problem.