How does this work with a rest arg?

```js
function f(n, ...a, #b, #c = 1) {
return [n, a, b, c, f(n + 10, ...a.map(x => x + 10), 'b', 'c')];
}
assert.equal(f.length, 1);
f(1, 2, 3, 4) // ?
```

It seems unacceptable to me to ban mixing a rest arg with a private arg; what would the output here be for this function call assuming it worked?

Start with the function example and call in the issue, then determine how a rest argument and private arguments should interact. Done means the proposal has a clear, agreed-upon answer for the call's behavior and whether this combination is supported.

Written by the indexing model from the issue text.