# File lib/poll.rb, line 268
        def setCallback( io, callback=nil, *args, &block )
                raise ArgumentError, "Handle #{io.inspect} is not registered" unless
                        @masks.has_key?( io )

                rv = nil
                if @callbacks.has_key?( io )
                        rv = @callbacks[ io ][:callback]
                end

                if callback || block
                        @callbacks[ io ] = { :callback => (callback || block), :args => args }
                else
                        @callbacks.delete( io )
                end

                return rv
        end