static <A> boolean isReflectiveTypeAdapter(TypeAdapter<A> adapter) {
    Preconditions.checkNotNull(adapter, "adapter");
    return adapter instanceof ReflectiveTypeAdapterFactory.Adapter;
  }
Appends a TypeAdapterFactory to allow Gson and Bson adapters to co-exist.
Avoids registering the TypeAdapterFactory if Gson already has factories for BSON types.

@param gson preconfigured instance
@return {@code gson} configured with delegating type adapter factory.
mongo/src/org/immutables/mongo/bson4gson/GsonCodecs.java