code: def _to_string(self):
          
          # Allow access to _to_string protected method
          return u"{course_key}+{BLOCK_TYPE_PREFIX}@{block_type}+{BLOCK_PREFIX}@{block_id}".format(
              course_key=self.course_key._to_string(),  # pylint: disable=protected-access
              BLOCK_TYPE_PREFIX=self.BLOCK_TYPE_PREFIX,
              block_type=self.block_type,
              BLOCK_PREFIX=self.BLOCK_PREFIX,
              block_id=self.block_id
          )
docs: Return a string representing this location.