code: def _check_unpack_options(extensions, function, extra_args):
      
      # first make sure no other unpacker is registered for this extension
      existing_extensions = {}
      for name, info in _UNPACK_FORMATS.items():
          for ext in info[0]:
              existing_extensions[ext] = name
  
      for extension in extensions:
          if extension in existing_extensions:
              msg = '%s is already registered for "%s"'
              raise RegistryError(msg % (extension,
                                         existing_extensions[extension]))
  
      if not isinstance(function, collections.Callable):
          raise TypeError('The registered function must be a callable')
docs: Checks what gets registered as an unpacker.