The scheme option description unclear

It states:

> `scheme` - allows adding or removing the scheme part for a URI
>
> ```
> ${'//example.com/path/page.html' @ scheme='http'}
>
>
> ${'http://example.com/path/page.html' @ scheme='https'}
>
>
> ${'http://example.com/path/page.html' @ scheme=''}
>
>
> ${'http://example.com/path/page.html' @ scheme}
>
> ```

It looks like it doesn't remove scheme, even in examples above. I checked Sightly implementation and it also doesn't remove scheme.
I assume that removed scheme means the following output:
> ```
> ${'http://example.com/path/page.html' @ scheme=''}
>
> ```

Could you please clarify it?