@Override
  public synchronized void init(ProcessingEnvironment processingEnv) {
    env.initProcessing(processingEnv);
    super.init(processingEnv);
  }
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