Function transparent_clones 

fn transparent_clones<I, G>(
    source: &[Transformed<Node<I, G>, G>],
    len: usize,
) -> Vec<Transformed<Node<I, G>, G>>
where
    I: Opacity + Clone,
    G: Clone,

Grow source to len entries by cloning entries and marking every clone transparent — the absence rule of Alignable::align_with for child lists that have nothing of their own to repeat.