Generate SEQRES lines representing the contents

def seqres_lines(self):
    """Generate SEQRES lines representing the contents"""
  
    lines = []
  
    for chain in self.keys():
      seq = self[chain]
      serNum = 1
      startidx = 0
      while startidx < len(seq):
        endidx = min(startidx+13, len(seq))
        lines += ["SEQRES  %2i %s %4i  %s\n" % (serNum, chain, len(seq), " ".join(seq[startidx:endidx]))]
        serNum += 1
        startidx += 13
    
    return lines