code: def _check_source(cls, source_file_hash, source):
          
          embedding_name = cls.__name__.lower()
          if source not in source_file_hash:
              raise KeyError('Cannot find pre-trained source {} for token embedding {}. '
                             'Valid pre-trained file names for embedding {}: {}'.format(
                                 source, embedding_name, embedding_name,
                                 ', '.join(source_file_hash.keys())))
docs: Checks if a pre-trained token embedding source name is valid.
  
  
          Parameters
          ----------
          source : str
              The pre-trained token embedding source.