old_contents: def regress_course(course):
      course['depts'] = course['departments']
      del course['departments']
      course['desc'] = course['description']
      del course['description']
      course['num'] = course['number']
      del course['number']
      course['pf'] = course['pn']
      del course['pn']
  
new_contents: def regress_course(course):
      if 'departments' in course:
          course['depts'] = course['departments']
          del course['departments']
      if 'description' in course:
          course['desc'] = course['description']
          del course['description']
      if 'number' in course:
          course['num'] = course['number']
          del course['number']
      if 'pn' in course:
          course['pf'] = course['pn']
          del course['pn']
  
subject: check for keys before regressing a course