docs: public SelKey register(ChannelSelector sel, Op ops, Object att)
      {
          if (selector != null)
          {
              throw new IllegalStateException("already registered with "+selector);
          }
          if (validOp != ops)
          {
              throw new IllegalArgumentException("expected "+validOp+" got "+ops);
          }
          key = sel.register(this, ops, att);
          selector = sel;
          return key;
      }