code: public final SelectionKey register(Selector sel, int ops)
          throws ClosedChannelException
      {
          return register(sel, ops, null);
      }
docs: Registers this channel with the given selector, returning a selection
  key.
  
  <p> An invocation of this convenience method of the form
  
  <blockquote><tt>sc.register(sel, ops)</tt></blockquote>
  
  behaves in exactly the same way as the invocation
  
  <blockquote><tt>sc.{@link
  #register(java.nio.channels.Selector,int,java.lang.Object)
  register}(sel, ops, null)</tt></blockquote>
  
  @param  sel
  The selector with which this channel is to be registered
  
  @param  ops
  The interest set for the resulting key
  
  @throws  ClosedChannelException
  If this channel is closed
  
  @throws  ClosedSelectorException
  If the selector is closed
  
  @throws  IllegalBlockingModeException
  If this channel is in blocking mode
  
  @throws  IllegalSelectorException
  If this channel was not created by the same provider
  as the given selector
  
  @throws  CancelledKeyException
  If this channel is currently registered with the given selector
  but the corresponding key has already been cancelled
  
  @throws  IllegalArgumentException
  If a bit in <tt>ops</tt> does not correspond to an operation
  that is supported by this channel, that is, if {@code set &
  ~validOps() != 0}
  
  @return  A key representing the registration of this channel with
  the given selector