docs: @NonNull
    public static <K, V> ImmutableMap<K, V> append(
        @NonNull ImmutableMap<K, V> map, @NonNull K newKey, @NonNull V newValue) {
      ImmutableMap.Builder<K, V> builder = ImmutableMap.builder();
      for (Map.Entry<K, V> entry : map.entrySet()) {
        if (!entry.getKey().equals(newKey)) {
          builder.put(entry);
        }
      }
      builder.put(newKey, newValue);
      return builder.build();
    }