-(NSMutableArray *)operandStack // override the getter for lazy instantiation
{
    if (!_operandStack)
    {
        [self setOperandStack:[[NSMutableArray alloc] init]];
    }

    return _operandStack;
}


-(void)pushOperand:(double)operand
{
    NSNumber *operandObject = [NSNumber numberWithDouble:operand];
    [[self operandStack] addObject:operandObject];
}


-(double)popOperand
{
    NSNumber *operandObject = [[self operandStack] lastObject];

    if (operandObject)
    {
        [[self operandStack] removeLastObject];
        return [operandObject doubleValue];
    }
    else
    {
        return 0;
    }
}