value-fixture/src/org/immutables/fixture/encoding/defs/OptionalMap.java
@Encoding.Expose
  public Optional<Map<K, V>> get() {
    return Optional.<Map<K, V>>ofNullable(map);
  }
@Encoding.Expose
  public Optional<Map<K, V>> get() {
    return Optional.<Map<K, V>>ofNullable(map);
  }
Expose (and detect) the field as Optional of Map