static HashSet<S> requiredPhraseImageMD5s() {
  new HashSet<S> md5s;
  for (S s : scanLog_iterator(glyphTeachLog())) {
    s = afterSquareBracketStuff(s);
    Pair<S> p = splitAroundColon_toPair(s);
    if (p == null) continue;
    addUnlessNull(md5s, regexpFirstGroupIC("Image (" + md5Regexp() + ")", p.a);
  }
  ret md5s;
}