public bool Validate()
{
if (_digits.Length != 10)
return false;
int sum = 0;
int factor = 10;
for (var i = 0; i < 9; i++) {
sum += _digits[i]*factor;
factor--;
}
var checkDigit = 11 - (sum%11);
if (checkDigit == 11) checkDigit = 0;
return checkDigit == _digits[9];
}