*         sufficient free space.
     */
    private int allocTail(int size) {
        size = (size + ~TAILBLOCK_MASK) & TAILBLOCK_MASK;
        int alloc = _alloc - size;
        if (alloc >= _keyBlockEnd) {
            _alloc = alloc;
            return alloc;
        } else {
            return -1;
        }
    }