_M_copy
__top->_M_right = _M_copy(_S_right(__x), __top);
__y->_M_right = _M_copy(_S_right(__x), __y);
_Link_type _M_copy(_Link_type __x, _Link_type __p);
_M_root() = _M_copy(__x._M_root(), _M_header);
_M_root() = _M_copy(__x._M_root(), _M_header);