code: def maskQuality(self, umi, umi_quals):
          '''mask low quality bases and return masked umi'''
          masked_umi = mask_umi(umi, umi_quals,
                                self.quality_encoding,
                                self.quality_filter_mask)
          if masked_umi != umi:
              self.read_counts['UMI masked'] += 1
              return masked_umi
          else:
              return umf maskQuality(self, umi, umi_quals):
          '''mask low quality bases and return masked umi'''
          masked_umi = mask_umi(umi, umi_quals,
                                self.quality_encoding,
                                self.quality_filter_mask)
          if masked_umi != umi:
              self.read_counts['UMI masked'] += 1
              return masked_umi
          else:
              return umi
docs: mask low quality bases and return masked umi