shadow_child
r = shadow_child(s->info, vt, pn, parent_index + 1, &right);
r = shadow_child(s->info, vt, pn, parent_index - 1, &left);
r = shadow_child(s->info, vt, parent, parent_index - 1, &sib);
r = shadow_child(s->info, vt, parent, parent_index + 1, &sib);