docs: def fastq(self):
          '''
          str: Returns the sequence, as a FASTQ-formatted string
  
          If ``Sequence.qual`` is ``None``, then ``None`` will be returned instead of a
          FASTQ string
          '''
          if self.qual is None:
              self._fastq = None
          else:
              if self._fastq is None:
                  self._fastq = '@{}\n{}\n+\n{}'.format(self.id, self.sequence, self.qual)
          return self._fastq