// assumes map never contains null values
static <A, B> AutoCloseable tempMapPutAll(Map<A, B> map, Map<A, B> toAdd) {
  if (map != null && nempty(toAdd)) {
    new LPair<A, B> toRestore;
    for (key, value : toAdd) {
      if (key != null && value != null) {
        B old = map.put(key, value);
        if (!eq(old, value))
          toRestore.add(pair(key, old));
      }
    }
    ret -> {
      for (p : toRestore)
        mapPutOrRemove(map, p.a, p.b);
    };
  }
  null;
}

Began life as a copy of #1034293