private BsonType advance() {
    return delegate.readBsonType();
  }
In order to be able to get current location of the BSON "cursor" (document or array start/end)
enforcing clients to
provide {@link AbstractBsonReader} which exposes this information via {@code getState()}
method, instead of more generic interface {@link BsonReader}.
@param delegate
@see AbstractBsonReader#getState()
mongo/src/org/immutables/mongo/bson4gson/BsonReader.java