@SuppressWarnings("CheckReturnValue")
  void use() {
    HiddenImplementation instance = new HiddenImplementationBuilder().build();
    instance.toString();
  }
Feature combination
<ul>
<li>Private implementation
<li>Outside builder
<li>Builder returns abstract
</ul>
value-fixture/src/org/immutables/fixture/style/HiddenImplementation.java