    void operator()(User* user) {
        // Remove the user from the borrower if it hasn't already been swapped out.
        if (_borrower && *_it == user) {
            fassert(40546, _borrower->removeAt(_it) == user);
        }
        _owner->releaseUser(user);
    }
    void operator()(User* user) {
         
        if (_borrower && *_it == user) {
            fassert(40546, _borrower->removeAt(_it) == user);
        }
        _owner->releaseUser(user);
    }