reqs[rank][vdir].ledger_extent_.unite (ledger_extent);
reqs[rank][vdir].head_extent_.unite (head_extent);
reqs[rank][vdir].position_
reqs[rank][vdir].ledger_extent_.unite (ledger_extent);
reqs[rank][vdir].head_extent_.unite (head_extent);
reqs[rank][vdir].position_