static Map getMapOpt(Map map, O key) {
  ret map == null ? null : (Map) map.get(key);
}

static Map getMapOpt(L l, int idx) {
  ret (Map) get(l, idx);
}

static Map getMapOpt(O o, O key) {
  if (o instanceof Map) ret getMapOpt((Map) o, key);
  if (key instanceof S)
    ret (Map) getOpt(o, (S) key);
  throw fail("Not a string key: " + getClassName(key));
}

Began life as a copy of #1003889