usage of the method of exchange

[3625365git-tx]

page 56 of the book, example 3-4
how to explain the exchange process referring to "input1.exchange(input2,0x1,output.bits( ) )"?

from the docs, i learned that this method allows one or two arguments,so why three here?
besides, to my understanding, the output as the control_qubit, isn't supposed to change after the filled dot, so the readout should
always be 1, whether the two inputs are equal of not.
could you please explain my questions? thanks a lot!

[3625365git-tx]

fine,solved myself