na_seq = NArray.to_na(self.seq, "byte")
na_qual = NArray.to_na(self.qual, "byte")
mask = (na_qual - SCORE_ILLUMINA) < cutoff
+ mask *= na_seq.ne("-".ord)
na_seq[mask] = 'N'.ord
na_seq = NArray.to_na(self.seq, "byte")
na_qual = NArray.to_na(self.qual, "byte")
mask = (na_qual - SCORE_ILLUMINA) < cutoff
+ mask *= na_seq.ne("-".ord)
na_seq[mask] ^= ' '.ord