Before revision: In order to help understand the circuit implementations below, it is convenient to think of NAND, NOR, AND and OR as controlled operations .Namely, although all of these gates are symmetric in their input , it is convenient to pick one bit as a control bit set by our choice, and the other bit as the input to be processed depending on the state of the control.
After revision: It is convenient to think of NAND, NOR, AND and OR as controlled operations , where one input is chosen as the control input set and the other bit as the input to be processed depending on the state of the control.