Combined logics
If you invert an OR afterwards, it 'logically' will be a NOR.
And if I invert the signals before the OR, will it be the same?
A common situation at educating a child: "When you talk or make a mess during supper, you will get no pudding afterwards and go to bed immediately."
"If you don't talk or don't make a mess..." then the until now silent, totally stained but clever child will cry: "I want my pudding! I had not said any word yet!" Make it better next time by saying "If you don't talk and don't make a mess..." You see: If NOT A AND NOT B Then is the same as IF A OR B Then NOT - even neither A NOR B.
move on