static MultiMap<S, S> getLearnedChars() {
  ret getLearnedChars(getLearnedChars_fullImages());
}

static MultiMap<S, S> getLearnedChars(MultiMap<BWImage, S> map) {
  new MultiMap<S, S> md5ToChar;
  for (BWImage img : map.keySet())
    md5ToChar.putAll(md5OfBWImage(img), map.get(img));
  ret md5ToChar;
}