// TODO: check validity
static S extractYouTubeID(S s) {
  if (empty(s)) null;
  int i = indexOfAfter(s, "youtu.be/");
  if (i >= 0)
    ret substring(s, i);
  s = substring(s, indexOf(s, '?')+1);
  try {
    try answer mapGet(decodeHQuery(s), "v");
  } catch {}
  ret s;
}