/sci/ - Science & Math

Can someone give me a hint on how to prove reverse inclusion? I've been messing with this for a while and nothing's popping out.
My original plan was to just set up a net in S converging to an arbitrary point in the intersection, but it seems like getting something well-defined requires essentially assuming that point is a limit point of S in the first place. Trying to get a contradiction by assuming there's a neighborhood of a point of the intersection which is disjoint from S hasn't gotten me anywhere either.