code: def get_offset(cls, info):
          
  
          assert info.layer == 3
  
          if info.version == 1:
              if info.mode != 3:
                  return 36
              else:
                  return 21
          else:
              if info.mode != 3:
                  return 21
              else:
                  return 13
docs: Calculate the offset to the Xing header from the start of the
          MPEG header including sync based on the MPEG header's content.