code: def _common(s1, s2, i1, i2):
      
      c = len(set(s1).intersection(s2))
      t = min(len(s1), len(s2))
      pct = 1.0 * c / t * t
      is_gt = up_threshold(pct, t * 1.0, parameters.similar)
      logger.debug("_common: pct %s of clusters:%s %s = %s" % (1.0 * c / t, i1, i2, is_gt))
      if pct < parameters.similar and is_gt and pct > 0:
          pct = parameters.similar
      return pct / t
docs: calculate the common % percentage of sequences