Function hierarchy_iter_depth_first_parent_to_child 

pub fn hierarchy_iter_depth_first_parent_to_child<R, F>(
    q: &EntitiesHolderQuery<'_, (EntityId, Option<&ChildComponent>, Option<&ParentComponent>), F>,
    root: Option<EntityId>,
    f: impl FnMut(EntityId, &[EntityId], Option<&R>) -> Option<R>,
)
where
    F: QueryFilter,

f(optional parent, children, cache) -> cache