@Override
    public final boolean run(Context<Object> context) {
      V value = extractor.get(context);
      @SuppressWarnings("unchecked")
      B builder = (B) context.getValueStack().peek();
      specify(builder, value);
      return true;
    }
Parboiled extraction for the sake of AST building.
trees/src/org/immutables/trees/ast/Extractions.java