Function reading_from_mouth 

pub fn reading_from_mouth<R, FnR>(mouth: Mouth, reader: FnR) -> Result<R>
where
    FnR: FnOnce() -> Result<R>,

Do something, while reading tokens from a specific Mouth.

This reads ONLY from that mouth (or any mouth openned by code in that source), and the mouth should end up empty afterwards, and only be closed here.