Function extended 

pub fn extended<B: Copy + Send>(
    out: &mut Vec<B>,
    n: usize,
    unit: usize,
    write: &(dyn Fn(Range<usize>, &mut Slots<'_, B>) -> bool + Sync),
) -> bool

Appends n items to out, and gives whether every call of write said true. write(range, slots) fills slots with the items in range, in order. The calls split across threads as split splits a call, in units of unit items. A call that leaves any of its slots unfilled panics, and out keeps its length.