Proposal static type constraints features

[YU HengChun]

> ```js
> function CustomNumber(x = void([Number, mod.BigNumber]) || 42) {
>   // ...
> }
> ```
> 
> which is arguably awkward.

Easy, change the word well.

```js
function CustomNumber(x = typeof([Number, mod.BigNumber]) && 42) {
  // ...
}
```