Hi, I'm trying to clip the first 20 bp of a series of fastq reads. For some reason, I have no problem using subseq() on the object returned by sread() but can't do it on the quality() object

 > fq
 class: ShortReadQ
 length: 1000000 reads; width: 151 cycles
 > subseq(sread(fq),21)
   A DNAStringSet instance of length 1000000
           width seq
       [1]   131 TTTTAAATAGGCGTTTTATTTTATTAAAGTG...ATTATTTGTATGAAGATCGTAATTTATTTT
       [2]   131 AGAGTGTGTGTAAGATGAAGGTTTTAAAAAG...TTCCTCCCAGATCGGAAGAGCACACGTCTG
       [3]   131 GGGTTTTGATTTTGAGATTGTATAATTTTGT...TATATATATATATATATATATATTTTATAT
       [4]   131 ATATTTTTTTATAGTGTTGAAAGTTTTTATA...AAATAAATTAAGTTTGTTTGTTTTAGATTT
       [5]   131 AAAATTATTTAGTAGGGATATTTTTTTAATT...GGGGATATGTAGGTTATTAGTATTTAGGAA
       ...   ... ...
  [999996]   131 TAAGAAGGGGTTATGTGATTTTTGATGGTGT...TATTATAAGGTTTTTTTAAAATGTTTTATG
  [999997]   131 TTTTGTGTTGTGTGTATTTAATTGATAGAGT...GTTTTGGGGTTAAAGGTAGAAAAGGAAATA
  [999998]   131 TTGAGTAGTATTTTATGGTATGTATGTATAT...TAATTTTTTTATTTATTTATTTATTGATTG
  [999999]   131 ATTTAGGGTATTGAATTAGGTAAGTTAGAGA...TTTAGAGATAAATTGTTTTAGTTTTTTTAT
 [1000000]   131 TTTTTTTTGATTTTTATTTGGTGGGGTTGGT...GGTTTTGTATAGTGTTTTTATTTGTGGGTT
!> subseq(quality(fq),21)
 Error in (function (classes, fdef, mtable)  :
   unable to find an inherited method for function ‘subseq’ for signature ‘"FastqQuality"’

is there something I'm missing?

narrow(quality(fq), 21)