I have also left out the padding and parsing of the second number. Again, I'll add that later. Right now I'm trying to walk you through the primary algorithims needed to make a math function. As an aside, I have used the same fundamental approach for a generalized parsing procedure that starts with the left-most character and proceeds to the right. This procedure is found by following this link .
Lets add two digits together using a number line.
Okay, I admit it. I don't know how I come up with this stuff. Anyway, the really important part can be understood if the ninth and tenth lines make sense. The ninth line sets a very special environment variable. One named for the first of the two digits to be added.