"macro" expansion to build switch case code

[Paul via Digitalmars-d-learn]

I have a struct similar to the following example.  I'd like to 
build an adder method without having to code the whole method.  
How do I use the D language to do this?  Template, mixins, 
CTFE..all of them?

```d
struct myS {
    int a, b, c, d, e, f, g, h, i;
    adder(string s, int n) {
        final switch (s) {
            case "aa" :
                a += n; break;
            case "bb" :
                b += n; break;
           ...
```
Thanks for any assistance.