Editable representable functors

In general,

```haskell
ixSureDef :: (Representable f, Eq (Rep f)) => Rep f -> Lens' (f a) a
ixSureDef i f fa = (\x' -> Rep.tabulate (\k -> if k == i then x' else Rep.index fa k)) <$> f (Rep.index fa i)
```

Of course, this is horrifically inefficient. Is there a good place here for a subclass supporting such an operation? Alternatively (or additionally), is there room in lens for an `IxSure` class similar to `Ixed` but providing a `Lens` instead of a `Traversal`?