Why not to use `mod` version from lens?
```javascript
var _val = c.get(a);
    if (!Array.isArray(_val)){
      return c.set(a, f(_val)); 
    } 
    else {
      return setter(a, _val.map(f));
    }
```
It's usefull for traverse. 
You needn't add map manualy, like it did in tests https://github.com/Suor/tscope.js/blob/master/test/tests.js#L146