@Override
  public SourceVersion getSupportedSourceVersion() {
    @Nullable SupportedSourceVersion sourceVersion = this.getClass().getAnnotation(SupportedSourceVersion.class);
    if (sourceVersion != null) {
      return sourceVersion.value();
    }
    return SourceVersion.latestSupported();
  }
Override process method and call {@link #invoke(org.immutables.generator.Templates.Invokable)}
from inside it passing invokable fragments from generated template instances.
generator/src/org/immutables/generator/AbstractGenerator.java