code: def query_unique(self, field):
          
          return set(self._convert_field(field, value)
                     for value in super(Reader, self).query_unique(field))
docs: Return a list of unique values appearing in the journal for the given
          `field`.
  
          Note this does not respect any journal matches.
  
          Entries will be processed with converters specified during
          Reader creation.