docs: def sort_header(header_text):
    """sort the chromosomes in a header text"""
    lines = header_text.rstrip().split("\n")
    rlens = {}
    for ln in lines:
      m = re.match('@SQ\tSN:(\S+)\tLN:(\S+)',ln)
      if m:
        rlens[m.group(1)] = m.group(2)
    output = ''
    done_lens = False
    for ln in lines:
      if re.match('@SQ\tSN:',ln):
        if not done_lens:
          done_lens = True
          for chr in sorted(rlens.keys()):
            output += "@SQ\tSN:"+chr+"\tLN:"+str(rlens[chr])+"\n"
      else:
        output += ln.rstrip("\n")+"\n"
    return output