static O[] paramsPlus(O[] a1, O... a2) {
  if (a2 == null || l(a2) == 2 && a2[0] == null) ret a1;
  if (a1 == null) ret a2;
  assertEvenLength(a1);
  assertEvenLength(a2);
  Map map = paramsToOrderedMap(a1);
  int n = l(a2);
  for (int i = 0; i < n; i += 2) {
    O key = a2[i];
    if (key != null) map.put(key, a2[i+1]);
  }
  ret mapToParams(map);
}