webMethods contains several useful built in services in the pub.math folder. One problem with these services though, is that they are not "safe". For instance if you call pub.math:addInts and pass in the values 1 and null it throws an error. This means that instead of assuming that a null is zero, you need to introduce the overheads of extra error handling.
Write your own "Safe" math classes. The package contains services in the "safemath" folder that cleanly wrap up the functionality and can make mapping chores just that little bit more maintainable.
A simple two line solution for each of the services you wish to wrap up for safety is to put a MAP step which sets num1, num2 to 0 with the "Overwrite pipeline variable" checkbox de-selected: