@Nullable
  <K, V> UpdatableIndex<K, V, FileContent, ?> combineIndex(@NotNull FileBasedIndexExtension<K, V> indexExtension,
                                                           @NotNull UpdatableIndex<K, V, FileContent, ?> baseIndex) throws IOException;
@Nullable
  <K, V> UpdatableIndex<K, V, FileContent, ?> combineIndex(@NotNull FileBasedIndexExtension<K, V> indexExtension,
                                                           @NotNull UpdatableIndex<K, V, FileContent, ?> baseIndex)