Function width 

pub fn width<'a, I>(srcs: I) -> usize
where
    I: Iterator<Item = &'a [u8]>,

How long the result of op over srcs will be.

As long as the longest source, since a shorter one reads as zeros past its end. A caller sizes its destination with this and then fills it with combine.

Panics

If srcs is empty, which is refused with a message on the wire.