if (_diff < _toWithdraw) {
    // @audit burns too many shares for a below fair-share amount
    _amount = _balance.add(_diff);
}
if (_diff < _toWithdraw) {
    _amount = _balance.add(_diff);
    // recompute _shares to burn based on the lower payout
    // should be something like this, better to cache balance() once at the start and use that cached value
    _shares = (totalSupply().mul(_amount)).div(_balance);
}
