CHECK(!frozen_);
    if (!IsCopy() && Contains(resource_op)) {
      // We can avoid the copy if the item we want to insert already exists.
      return;
    }

    EnsureIsCopied();
    impl_->insert(resource_op);
  }

  Impl::const_iterator begin() const {
    return impl_ ? impl_->begin() : GetEmptyImpl()->begin();
  }

  Impl::const_iterator end() const {