value-fixture/src/org/immutables/fixture/encoding/defs/OptionalMap.java
@Encoding.Expose
  public Optional<ImmutableMap<K, V>> getImmutable() {
    return Optional.ofNullable(map);
  }
@Encoding.Expose
  public Optional<ImmutableMap<K, V>> getImmutable() {
    return Optional.ofNullable(map);
  }
alternatively it can be exposed as Optional of ImmutableMap