Thanks for contrexample.
I think that behavior of `mod` should depends on what is using `Lens` or `Traverse`.
```javascript
var data = {colors: ['red', 'green']};
var lens = Tscope.attr('colors');
lens.mod(function (xs) { return xs.concat(['blue']) } //{colors: ['red', 'green', 'blue']};
```
```javascript
var data = {colors: ['red', 'green']};
var traverse = Tscope.traversed(Tscope.attr('colors'));
traverse.mod(function (x) { return x.toUpperCase(); }  // {colors: ['RED', 'GREEN']};
```
I think that separate type for `Traverse` are needing. And it's exist in original haskell implementation.