docs: def _compute_threshold(self,z=2.0):
          """
          m._compute_threshold(z=2.0) -- For Motif objects assembled from a set of sequence,
                                         compute a self.threshold with a z-score based on the distribution
                                         of scores in among the original input sequences.
          """
          scoretally = []
          for seq in self.seqs:
              matches,endpoints,scores = self.scan(seq,-100)
              scoretally.append(scores[0])
          ave,std = avestd(scoretally)
          self.threshold = ave - z *std