Chapter 4 Exercise 5

Thanks for writing this great book. I'm new to TypeScript and I'm enjoying working through it.

I noticed that the answer to Chapter 4 Exercise 5 has the function signature:

```typescript
function is(a: T, ...b: [T, ...T[]]): boolean
```

However, when I was attempting it on my own, I came up with:

```typescript
function is(a: T, ...b: T[]): boolean
```

which also seems to work. It looks like there's also an open PR (#8) with the signature:

```typescript
function is(a: T, ...b: [...T[]]): boolean
```

What's the difference, if any, between these different generic types?

Start with the Chapter 4 Exercise 5 answer and compare its displayed signature with the alternatives in the issue and open PR #8. Done means the answer clearly explains the distinction between these generic parameter forms and identifies the appropriate signature.

Written by the indexing model from the issue text.