Question: exceptionDuringOperation::: and NSDecimalNumberHandler protocol

[[I code:forFun]]

Hello : )

While working with the NSDecimalNumber class, I've implemented a few functions to add and subtract numbers.
I'd like to properly implement the NSDecimalNumberBehaviors protocol.

My 'calculator' class has an addition method:

Code: (Objective-C)

-(NSDecimalNumber *)addNum1:(NSDecimalNumber *)num1 andNum2:(NSDecimalNumber *)num2
{
    NSDecimalNumber *result = [num1 decimalNumberByAdding:num2 withBehavior:_behavior];
    return result;
}

_behavior is defined beforehand (in the class's init method) as follows:

Code: (Objective-C)

_behavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundPlain scale:NSDecimalNoScale raiseOnExactness:YES raiseOnOverflow:YES raiseOnUnderflow:YES raiseOnDivideByZero:YES];;

Apple's documentation states that three methods are required to implement the NSDecimalNumberBehaviors protocol as seen here:
-rounding mode
-scale
– exceptionDuringOperation:error:leftOperand:rightOperand:

My question is, can someone provide an example of how to properly conform to the protocol?  More specifically, I'm trying to figure out whether the (exceptionDuringOperation:error:leftOperand:rightOperand:) method needs to be included within my (addNum1:andNum2:) method somehow, or is it supposed to be a separate method within the 'calculator' class.  What would this class look like with the protocol properly implemented?  I'm not getting the picture yet.

Have I provided enough information to answer this question?  (I have the 3rd edition, not with me at the moment : ), and am working through it to become knowledgeable).  Thanks!