code: public Boolean visit(final CompilationUnit n, final Object arg) {
          final Boolean result = super.visit(n, arg);
          return result != null && result;
      }
docs: We need to override this method to manage the case where the visitor returns {@code null}. {@code null} is
  considered as {@code false}.
  
  @param n   the unit
  @param arg meaningless
  @return whether or not the compilation unit is a controller.