et V be an oracle circuit that marks one or more elements, acting as follows: V $(|$x$|$a$)=|$x$|$a $$ f $($x$)$

Here a takes the values $0$ or $1,$ and we have f$($x$)=1$ when x is the index of a marked element, and f$($x$)=0$ otherwise. How could V be altered to allow Grover$$s search to find an unmarked element?