code: def coverage(self):
          
          intervals = ReadIntervals(self.subjectLength)
          for hsp in self.hsps():
              intervals.add(hsp.subjectStart, hsp.subjectEnd)
          return intervals.coverage()
docs: Get the fraction of this title sequence that is matched by its reads.
  
          @return: The C{float} fraction of the title sequence matched by its
              reads.