code: def make_relative(cls, course_locator, block_type, block_id):
          
          if hasattr(course_locator, 'course_key'):
              course_locator = course_locator.course_key
          return course_locator.make_usage_key(
              block_type=block_type,
              block_id=block_id
          )
docs: Return a new instance which has the given block_id in the given course
          :param course_locator: may be a BlockUsageLocator in the same snapshot