For C$==1,$ the output $($called f$)$ is $1$ if P$0$ and P$1$ are equal $($P$0==$ P$1==1$ or P$0==$ P$1==0),$ and $0$ if not $($P$0!=$ P$1).$ For C$==0,$ the output is $1$ if P$0$ and P$1$ are not equal and $0$ if they are $($e$.$g$.$ the reverse behavior of C$==1).$