I had an instance when I was checking if two sets have elements in common:

```
if !a.intersection(b).isEmpty { ... }
```

when realised it would work quicker if I do it manually:

```
extension Set {
func containsMembers(from other: Set) -> Bool {
count < other.count ? contains { other.contains($0) } : other.contains { contains($0) }
}
}
```

As it would avoid making the intersection first (memory allocation and carefully calculating the whole subset of elements in common even when we know that a single common element is enough to know the outcome).

Could be a little but handy addition to the standard library.