static Set<S> aggressivelyCollectPossibleMD5sInTextFile(File f) {
  new HashSet<S> md5s;
  aggressivelyCollectPossibleMD5sInTextFile(f, md5s);
  ret md5s;
}

static void aggressivelyCollectPossibleMD5sInTextFile(File f, Set<S> out) {
  for (S s : linesFromFile(f))
    aggressivelyCollectPossibleMD5s(s, out);
}