SupplementedWave buildSupplement(ParticipantId viewer, ObservableConversationView conversations,
ObservableWaveletData udw) {
// Use mock state if there is no UDW.
PrimitiveSupplement udwState =
udw != null ? WaveletBasedSupplement.create(OpBasedWavelet.createReadOnly(udw))
: new PrimitiveSupplementImpl();
return SupplementedWaveImpl.create(udwState, conversations, viewer, DefaultFollow.ALWAYS);
}