Function difft::diff::sliders::unchanged_descendants_for_outer_slider

fn unchanged_descendants_for_outer_slider<'a>(
    nodes: &[&'a Syntax<'a>],
    found: &mut Vec<&'a Syntax<'a>>,
    change_map: &ChangeMap<'a>
)

Nested sliders require a single unchanged descendant whose delimiters we can slide.

(old-1 (novel (old-2)))

To slide, we want a single list that contains unchanged items but the outer delimiters are novel.

Find all the unchanged descendants.