docs: @NonNull
    public static <K, V> ImmutableMap<K, V> concat(
        @NonNull ImmutableMap<K, V> map1, @NonNull Map<K, V> map2) {
      ImmutableMap.Builder<K, V> builder = ImmutableMap.builder();
      for (Map.Entry<K, V> entry : map1.entrySet()) {
        if (!map2.containsKey(entry.getKey())) {
          builder.put(entry);
        }
      }
      builder.putAll(map2);
      return builder.build();
    }