sclass SRTEntry {
  int index;
  double start, end; // seconds
  S text;
  
  bool valid() {
    text = trim(text);
    ret !containsEmptyLines(text);
  }
  
  public S render aka toString() {
    assertTrue(valid());
    
    ret linesLL(
      index,
      srtFormatTime(start) + " --> " + srtFormatTime(end),
      text);
  }
}