static JsonReader readerAt(BsonValue value) throws IOException {
    BsonDocument doc = new BsonDocument("value", value);
    BsonReader reader = new BsonReader(new BsonDocumentReader(doc));
    // advance AFTER value token
    reader.beginObject();
    check(reader.peek()).is(JsonToken.NAME);
    check(reader.nextName()).is("value");
    return reader;
  }
Creates reader for position at {@code value}
mongo/test/org/immutables/mongo/bson4gson/Jsons.java