docs: protected boolean canDeliver(Address sender, long seqno) {
          BoundedHashMap<Long,Long> seqno_set=delivery_table.get(sender);
          if(seqno_set == null) {
              seqno_set=new BoundedHashMap<>(delivery_table_max_size);
              BoundedHashMap<Long,Long> existing=delivery_table.put(sender,seqno_set);
              if(existing != null)
                  seqno_set=existing;
          }
          return seqno_set.add(seqno, seqno);
      }