Function print 

pub fn print(
    dds: &Dds,
    printer: &Dd,
    job: &Job,
    notify: &mut dyn FnMut(Event<'_>),
) -> Result<(), String>

Prints job on printer: each file’s DD, read whole, appended to it in order. Every file needs a DD before anything prints, as lp prints nothing when it cannot read one. notify hears each DD opened and closed.