code: def iter_fasta_qual(fastafile, qualfile, defaultqual=OKQUAL, modify=False):
      
      from Bio.SeqIO.QualityIO import PairedFastaQualIterator
      if not qualfile:
          qualfile = make_qual(fastafile, score=defaultqual)
  
      rec_iter = PairedFastaQualIterator(open(fastafile), open(qualfile))
      for rec in rec_iter:
          yield rec if not modify else modify_qual(rec)
docs: used by trim, emits one SeqRecord with quality values in it