I guess we all know about destructuring to swap two variables `a` and `b`, namely: `[a, b] = [b, a]` and other clever ways like `a = b + (b = a, 0)`. However, I have a different situation which I feel should have a simple answer.

I have a variable `a` that can be one of `0, 1, 2, 3, 4, 5, or 6`. If it is `a===3` then I want `a` to reassign to `4`. If `a===4` then I want `a` to reassign to `3`. **BUT** if `a` is any of the other values, I want it to remain the same.

So is there a clever, 1 line bit of code that can do this? For completeness, I want a short version of this:

```
switch(a){
    case 3: a = 4; break;
    case 4: a = 3; break;
    // default: a = a; break; // Hashed out, because it is not needed
}

```