eval.c:6278
static VALUE
rb_mod_append_features(module, include)
    VALUE module, include;
{
    switch (TYPE(include)) {
      case T_CLASS:
      case T_MODULE:
	break;
      default:
	Check_Type(include, T_CLASS);
	break;
    }
    rb_include_module(include, module);

    return module;
}
