[NTG-context] Tracking overfull/underfull vboxes, widows/orphans

Hi, some time ago Hans has added these trackers for overfull/underfull hboxes: I'm wondering if there is something similar for vertical boxes? I've found these trackers: builders.vspacing builders.vpacking builders.page_vspacing builders.collect_vspacing But I don't really understand what they do. (Yes, I've tried them.) Maybe someone can shed some light on this ? (RTFM is a perfectly fine answer, just need to know which manual...) Then, would it be possible to add a tracker for widows/orphans? (Or even for prevented widows/orphans ?) Denis