[](const shared_ptr<TraceTreeItem> &a, const shared_ptr<TraceTreeItem> &b) {
const auto aext = a->v_extents();
const auto bext = b->v_extents();
[](const shared_ptr<TraceTreeItem> &a, const shared_ptr<TraceTreeItem> &b) {
const auto aext = a->v_extents();
const auto bext = b->v_extents();